WordNet sense 1 (the series of vertebrae forming the axis of the skeleton and protecting the spinal cord):

backbone, spine, rachis, spinal column, vertebral column

WordNet sense 2 (the posterior part of a human (or animal) body from the neck to the end of the spine):

dorsum

WordNet sense 3 (the protective covering on the front, back, and spine of a book):

binding, cover

WordNet sense 4 (a support that you can lean against while sitting):

backrest

WordNet sense 5 (give support or one's approval to):

endorse, indorse, second

WordNet sense 6 (be behind; approve of):

support, plump for, plunk for

WordNet sense 7 (place a bet on):

gage, game, punt, stake, bet on

WordNet sense 8 (not current or belonging to the present time):

noncurrent

WordNet sense 9 (establish as valid or genuine):

back up

WordNet sense 10 (located in or toward the back or rear):

rear, rearward

WordNet sense 11 (located at or near the back of an animal):

hind, hinder, hinder

WordNet sense 12 (at or near the hind end in quadrupeds or toward the spine in primates):

posterior

WordNet sense 13 (in answer):

in reply

WordNet sense 14 (at or to or toward the back or rear):

backward, backwards, rearwards