The Collaborative International Dictionary of English v.0.48:

Nates \Na"tes\, n. pl. [L., the buttocks.]
   1. (Anat.)
      (a) The buttocks.
      (b) The two anterior of the four lobes on the dorsal side
          of the midbrain of most mammals; the anterior optic
          lobes.
          [1913 Webster]

   2. (Zool.) The umbones of a bivalve shell.
      [1913 Webster]

WordNet (r) 3.0 (2006):

nates
    n 1: the fleshy part of the human body that you sit on; "he
         deserves a good kick in the butt"; "are you going to sit on
         your fanny and do nothing?" [syn: buttocks, nates,
         arse, butt, backside, bum, buns, can,
         fundament, hindquarters, hind end, keister,
         posterior, prat, rear, rear end, rump, stern,
         seat, tail, tail end, tooshie, tush, bottom,
         behind, derriere, fanny, ass]