"Scumbag," sounds like the kind of hokey insult that would get you laughed at if you used it. When it was used in a New York Times, it got protests from some older readers, because once upon a time it meant "a used condom." Think about every time you've seen Batman refer, in children's cartoon, to criminals as scum, and you'll begin to understand how obscenity evolves.
There are people who say that animals swear when they, for example, growl or gesture aggressively at people. Although no one could mistake such things for friendly gestures, showing anger isn't the same thing as swearing. Swearing is more complicated than just aggression. Swearing can be a form of affectionate teasing among friends, it can be a way of insulting someone, it can be a way of letting off steam or frustration, or a way of showing unbridled enthusiasm. The only thing all verbal obscenity has in common is the deliberate crossing of social norms. And this is why swear words are always changing.
The Newly Innocent Obscenities
Golly! Zounds! Gadzooks! These are the kind of things Captain Marvel would say. Almost any other superhero would be too mature for such, childish silly words. And yet, during Shakespeare's time, they made him one of the more edgy writers out there. They're not just random sounds, but contractions, meant to make absolutely shocking sentiments less outright obscene. Golly, zounds, and gadzooks were, in order, god's body, god's wounds, god's hocks, and even god's hooks. While thinking about the Almighty's ham hock region might offend a few people, each of these words are the kind of things now deemed perfectly innocent. This shows a huge shift in social mores since the time of the Shakespeare.
Religious obscenities, when half of Europe was at war with the other half over the right way to practice Christianity, were a big deal. Referring to God in the corporeal sense was a way to scandalize people. To take the Lord's name in vain was to go against explicit Biblical instructions. These were some of the more obscene concepts of the age, but today are the most mild swear words most people can think of. God, hell, damn, and, to some extent, Jesus Christ, are no big deal anymore. Most people use them.
Ironically, the reason they got a toe hold in current society is the same reason they were so scandalous a few centuries ago. They could be genuine swear words, but they could also be expressions of religious ideas. Far, far back in Simpsons history, there was a storyline about how the kids got a lesson on hell in Sunday School. When asked, afterwards, about what they learned, Bart replied, "Hell." When Marge scolded him, he told her that, no, they had learned about the literal hell, and kept saying hell over and over until Marge, tired of hearing a word she considered inappropriate when coming out of her son's mouth, said, "Bart, you're not in church anymore. Don't swear." The line between actual devotion and blasphemy is tougher to delineate than most censors, and most people, imagine. Eventually most English speakers just stopped trying to find it at all, and people saying things like, "Mother of God," just became a noncontroversial emotional outburst.
The Animals Diverge From Their Excrement
Other swear words, which managed to skate into acceptability under a protective barrier of literalism, are bitch and ass. Both of those started out as literal meanings - animals - and might have been used as insults in their own right in their time. Ass is actually two words blended together to become an obscenity. Ass, the swear word, started out as irs, which meant the back end of anything, not just animals. Over time it became arse, and eventually rounded out and emerged as an ass. The two words were so alike that it was easy to sneak some ass into everyday life. Who remembers the West Wing characters constantly calling each other "jackass," which, being a donkey, was perfectly okay. In the next few years the first part of the word was peeled away, with the understanding that an ass still meant donkey, but eventually everyone stopped kidding themselves and allowed it to be another mild swear word regularly said on TV.
Bitch started out, and remains, a female dog in breeding condition. From there its meaning expanded to anything female in breeding condition, and eventually it expanded to become promiscuous women, angry women, angry or promiscuous homosexual men, or anything "especially disagreeable." Sliding between the slightly sexual, the slight reference to sexuality, and the literal meaning of the word got bitch into general conversation, and most television shows. It also helps that being "especially disagreeable," rather than meek and accommodating, has become a point of pride for both women and male homosexuals, and so even at its most insulting, the word has lost the power to shock as society has moved on.
As for things like pissing and shitting, which is what bitches, asses, and all other animals do, they're old English words. At least one of which dates back to the King James Bible. (2 Ki 18:27 But Rabshakeh said unto them, Hath my master sent me to thy master, and to thee, to speak these words? hath he not sent me to the men which sit on the wall, that they may eat their own dung, and drink their own piss with you?) These words, though not refined back then, have gotten both more abstract and a little more outré. This is an example of the way culture can continually reach for more delicacy. In France, "toilet" used to mean a small towel, which was kept near the chamber pot. It also meant the act of cleansing oneself. Old books often use the phrase, "She spent some time making her toilet," which means grooming and preparing oneself for an event. "Toilet water" was a kind of light perfume. Since these actions happened in private, near a chamber pot, they were used as a euphemism for actually using that chamber pot. Eventually, the word came to mean the actual toilet itself, and not the things near it. After that, saying "I need to go to the toilet," became indelicate, and people had to come up with more abstracted ways of saying the same thing. Cycles like this made piss and shit, while more commonly used in society, more vulgar than they originally were.
Four Letters and Starting With F
And then there's the swear word that's held steady for half a millennium; fuck. It seemed to spring upon the landscape fully-formed, and already an obscenity. The first instance of use of the word "fuck," came from a satirical poem, written in Latin, in the year 1500. The line is referring to a group of friars, and runs like this: "Non sunt in coeli, quia gxddbov xxkxzt pg ifmk." If it suddenly starts looking like Kryptonian instead of Latin after the word quia, it's because it had to be written in code. Each letter of the word was swapped out for the letter following it in the alphabet. Remember that the alphabet was in a different order back then, and that Latin conjugates verbs differently, but gxddbov translates as "fuccant." The overall line states, "They are not in Heaven, since they fuck the wives of Ely." That is one racy poem!
The word was, and continued to be, the big daddy of all swear words in English for many years. It became one of the words that kept Lady Chatterley's Lover banned in plenty of places. The word was unutterable in polite company. It's still banned from most television stations and most print media.
Still, it has always been used, and its increasing popularity means that it's becoming less likely to be held back from media discourse. Lately things have been changing especially fast. The FCC lately had to change regulations about fining news stations that aired spontaneous utterings of "fuck," in their news footage. It was found that the word has come to be something people use to express their frustration, instead of solely referring to sex. Frustration is not obscene, so it's highly likely that fuck may be sliding its way into generally and even media acceptability. As soon as the word acquires tones that aren't exactly the literal and obscene meaning that it was originally used to convey, censors relax. They have to. As we've seen, it's too easy to play with language, hiding deeper meanings behind compound words, resetting context, and making words seem innocent. Is it only a matter of time before five hundred years of dirtiness becomes sanitized as a mere expression of frustration? And if so, what to do we say then?