The Collaborative International Dictionary of English v.0.48:

Rump \Rump\, n. [OE. rumpe; akin to D. romp trunk, body, LG.
   rump, G. rumpf, Dan. rumpe rump, Icel. rumpr, Sw. rumpa rump,
   tail.]
   1. The end of the backbone of an animal, with the parts
      adjacent; the buttock or buttocks.
      [1913 Webster]

   2. Among butchers, the piece of beef between the sirloin and
      the aitchbone piece. See Illust. of Beef.
      [1913 Webster]

   3. Fig.: The hind or tail end; a fag-end; a remnant.
      [1913 Webster]

   Rump Parliament, or The Rump (Eng. Hist.), the remnant of
      the Long Parliament after the expulsion by Cromwell in
      1648 of those who opposed his purposes. It was dissolved
      by Cromwell in 1653, but twice revived for brief sessions,
      ending finally in 1659.
      [1913 Webster]

            The Rump abolished the House of Lords, the army
            abolished the Rump, and by this army of saints
            Cromwell governed.                    --Swift.
      [1913 Webster]

   Rump steak, a beefsteak from the rump. --Goldsmith.
      [1913 Webster]

WordNet (r) 3.0 (2006):

rump
    n 1: the part of an animal that corresponds to the human
         buttocks [syn: hindquarters, croup, croupe, rump]
    2: fleshy hindquarters; behind the loin and above the round
    3: 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]

Moby Thesaurus II by Grady Ward, 1.0:

90 Moby Thesaurus words for "rump":
   Chateaubriand, afterglow, afterimage, backside, balance, beam,
   behind, blade roast, bottom, breast, brisket, butt, butt end,
   buttocks, candle ends, chaff, chuck, chuck roast, clod, cold cuts,
   croup, crupper, debris, derriere, detritus, end, fag end, fanny,
   filet mignon, filings, flank, fossil, haunches, hind end, holdover,
   husks, knuckle, leavings, leftovers, loin, nates, odds and ends,
   offscourings, orts, parings, plate, plate piece, posterior,
   pot roast, rack, rags, rear, rear end, refuse, relics, remainder,
   remains, remnant, residue, residuum, rest, rib roast, ribs, roach,
   roast, rolled roast, round, rubbish, ruins, rump roast, saddle,
   sawdust, scourings, scraps, shadow, shank, shavings, short ribs,
   shoulder, shoulder clod, sirloin, straw, stubble, stump, survival,
   sweepings, tenderloin, trace, vestige, waste