Contrary to popular belief, the word crap has nothing to do with the fact that Thomas Crapper helped invent the flush toilet. It's actually a very old word that comes from Latin! Crap wasn't used as a noun until 1898, when good ol' anthimeria effect changed the decades-older verb into a new form. Crap underwent a plethora of alterations in Middle English, taking the forms of crappe, crappen, crappys, and craps before finally settling on the modern variation. Before that, in Middle French, it was either crappe or chaffin, as a word meaning "chaff", the inedible protective casings of grain. The semantic connection here is both it and human excrement is discarded, unwanted, and cast off. This comes from Latin crappa (note the p/f confusion that's endemic to basically all languages), with the same meaning. The origin of crappa is unknown, but it might derive from Proto-Indo-European gep, meaning "mouth" (a connection of eating to grain). Curiously, the word crap was not introduced to America until World War I, when soldiers picked up the fad of saying it from British soldiers.
Adam Aleksic, a freshman studying linguistics at Harvard University, has been described as the internet's sixth most famous etymologist. He has disturbing interests in words, vexillology, geography, board games, limericks, and law, and he loves writing about himself in the third person.
The Etymology Nerd