This article is from the alt.usage.english FAQ, by Mark Israel firstname.lastname@example.org with numerous contributions by others.
"Crap" does not derive from Thomas Crapper. Thomas Crapper
(1837-1910) did exist and did make toilets. (At least 3 authors
have gone into print asserting he was a hoax, but you can see some
of his toilets at the Gladstone Pottery Museum, Uttoxeter Road,
Longton, Stoke-on-Trent, Staffordshire ST3 1TQ, U.K.; phone
+44 1782 311378), and also at the Science Museum in London. The
word "crap" was imported into English from Dutch in the 15th
century, with the meaning "chaff". It is recorded in the sense "to
defecate" from 1846; Thomas Crapper did not set up his business
until 1861. Also, Thomas Crapper did not "invent" the flush toilet
(the ancient Minoans had them); he merely improved the design.
The Crapper company lived on until 1966 -- 105 years in business.
See the article "Thomas Crapper: Myth & Reality" from the June 1993
issue of "Plumbing and Mechanical" at
<http://www.theplumber.com/crapper.html>. You can see some
photographs of Thomas Crapper at