The word crap, meaning excrement, is from an English dialectical word originally meaning chaff or the discarded husks of grain. The ultimate origin is not certain, but it could be related to the Old French crappe, meaning siftings or grain left on the floor of a barn, and the medieval Latin crappa. From Promptorium Parvulorum Sive Clericorum, Lexicon Anglo-Latinum Princeps, from c.1440:
Crappe, or gropys of corne, acus.
(Crap, or crapis of corne, acus.)
By the end of the 15th century, crap was being used to refer to the residue left by rendering or melting fat.
By the mid-19th century, it was being used to refer to excrement. From Swell’s Night Guide of 1846:
“Where’s the plant, cully?"..."Fenced, in a dunniken."..."What? Fenced in a crapping ken?”
And also this:
Which of us had hold of the crappy (sh-ten) end of the stick?
Now of course there is the famous tale that the word crap is taken from Thomas Crapper, the inventor of the flush toilet. Unlike the elusive Dr. Condom, Thomas Crapper did actually exist, but he neither invented the flush toilet nor bequeathed his name for posterity.
Crapper lived from 1837-1910. As we have seen, the word crap was being used for excrement as early as 1846, when Crapper was only nine years old. So we can dismiss his name as the origin of the word. His was the case of a person coincidentally being born with an appropriate name, an aptronym.
As to inventing the flush toilet, that’s a bit of hyperbole. Crapper was in the plumbing supply business and he manufactured and sold toilets, among other plumbing fixtures. For a while he owned the patent for the Silent Valveless Water Waste Preventer, a device that enabled the toilet to flush when the tank was half full. He did not invent it, however. That credit goes to a man named Albert Giblin.
But Crapper did put his name on the toilets he sold and with the name Crapper emblazoned on so many Victorian plumbing fixtures, we can assume that he did play a role in popularizing the word crap.
(Sources: Oxford English Dictionary, 2nd Edition)
Copyright 1997-2016, by David Wilton