Back means the rear of something.(noun)
An example of a back is the tail of an animal.
Back is defined as to support or to move in a backward motion.(verb)
The definition of back is in the rear.(adjective)
An example of back used as an adjective is a back yard which means a yard situated behind the house.
See back in Webster's New World College Dictionary
Origin: ME bak < OE baec; akin to ON bak, OHG bahho
Origin: ME bac < abac < OE on bæc, backward
Origin: Du bak < LL bacca, water bowl
Origin: after George Back (1796-1878), Arctic navigator
See back in American Heritage Dictionary 4
Origin: Middle English bak
Origin: , from Old English bǽc.
Origin: Dutch bak
Origin: , from French bac
Origin: , from Old French, boat
Origin: , from Vulgar Latin *baccus, vessel
Origin: , probably of Celtic origin.
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