the rear or back part or view of an object, person, scene, etc.; that part which is opposite the front.
rump; buttocks.

1350–1400; Middle English back syde; see back1, side1; compare Old English bæce backside Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
backside (ˌbækˈsaɪd)
1.  the back of something
2.  informal the buttocks

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Word Origin & History

late 15c., from back (adj.) + side. In the specific sense of "rump, buttocks" it is first recorded c.1500.
Online Etymology Dictionary, © 2010 Douglas Harper
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Example sentences
The party is an old-boys' club that is better at covering its backside than
  thinking ahead.
The backside hides a dedicated laptop pocket with its own sleeve.
While both cover your backside, one comes with a label of status that appeals
  to some and not to others.
One of the great revolutions in modern science rests on the elongated backside
  of a grotesque, mutant worm.
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