In the United States, the word "crapper" is a dysphemism for "toilet", although it is not clear if this has anything to do with Thomas Crapper. It has been reasonably suggested that U.S. soldiers stationed in England during World War I (some of whom had little experience with indoor plumbing) saw many toilets printed with 'T. Crapper' in the glaze and brought the word home as a synonym for 'toilet.' 'Crapper' remains an Americanism.
The noun crap is old in the English language, one of a group of words applied to discarded cast offs, like "residue from renderings" (1490s) or in Shropshire, "dregs of beer or ale", meanings probably extended from Middle English crappe "chaff, or grain that has been trodden underfoot in a barn" (c.1440), deriving ultimately from Late Latin crappa "chaff." As an occupational name, Crapper may be akin to Thresher.
The verb to crap, meaning "to defecate" is recorded since 1846, and according to one theory, the word "crapper" simply developed from this. According to another theory, Crapper's advertising of flush toilets caused his name to become a synonym for them, and people then assumed they were his invention.
The manhole covers with his company's name on them in Westminster Abbey are now a minor tourist attraction.
Adam Hart-Davis,Thunder, Flush and Thomas Crapper, addresses many of the myths surrounding Thomas Crapper and his inventions.Reference