The Collaborative International Dictionary of English v.0.48:

Anus \A"nus\, n. [L., prob. for asnus: cf. Gr. ? to sit, Skr.
   [=a]s.] (Anat.)
   The posterior opening of the alimentary canal, through which
   the excrements are expelled.
   [1913 Webster]

WordNet (r) 3.0 (2006):

anus
    n 1: the excretory opening at the end of the alimentary canal

Moby Thesaurus II by Grady Ward, 1.0:

46 Moby Thesaurus words for "anus":
   abdomen, appendix, asshole, blind gut, bowels, brain, bung,
   bunghole, cecum, colon, duodenum, endocardium, entrails, foregut,
   giblets, gizzard, guts, heart, hindgut, innards, inner mechanism,
   insides, internals, intestine, inwards, jejunum, kidney, kishkes,
   large intestine, liver, liver and lights, lung, midgut, perineum,
   pump, pylorus, rectum, small intestine, spleen, stomach, ticker,
   tripes, vermiform appendix, viscera, vitals, works