Generally, we only invent new words when we have new objects, or concepts, to describe. Eggcorn is a new word, but it describes a concepts as old as language. Learn about eggcorns, and share your favorite stories of them.
We praise language for its practicality and fluidity, but it's surprising how long it takes for some concepts to earn a word. Although they are part of everyone's experience, no one thinks to come up with a name for them. As a result, we have to resort to long explanations, which in this case would go like, "that thing where a person thinks they know a word, but it turns out they've been substituting a similar-sounding word, one that kind of makes sense, for the actual word." Instead of that mouthful, we can just say, "eggcorn."
The word eggcorn was coined in 2003, when the linguistics blog Language Log issued a call, requesting words that would describe the concept. Geoffrey Pullum, a linguistics professor, suggested "eggcorn." Eggcorn is itself an eggcorn. It was word that Pullum had seen a woman mistakenly use for "acorn." The key to eggcorns is that they make sense. An acorn is a seed for the oak tree, meaning it is like a grain of corn, and it is shaped like an egg. That made sense to the woman, where the random group of letters which spelled "acorn" did not. Another common eggcorn is "Old Timer's Disease," for Alzheimer's Disease. Alzheimer's generally strikes older people, and in its early stages resembles the condition of being a bit old, eccentric, and forgetful.
The word eggcorn is part of a sometimes overlooked part of linguistics - how people screw up language. This area includes malapropisms, another kind of language substitution. Miss Malaprop was a fictional character whose constant misuse of language included phrases like, "He is the very pineapple of politeness." Malapropisms differ from eggcorns because they make so little sense, while eggcorns are logical substitutions, even if they are incorrect. Universal linguistic mistakes also include mondegreens, which are misheard song lyrics. These tend to be phrases, rather than single words.