Do you fear Friday the 13th? Well, you'd better stay inside today and nurse your paraskevidekatriaphobia, which is the official term - derived from Greek - meaning "fear of Friday and thirteen." Apparently the phobia was originally called friggatriskaidekaphobia, but that pissed off language nerds because it combined a Norse root (frigga, for Friday) with the Greek phobia. Let Language Log break it all down for you.