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1868, back panel on a wagon, hinged to swing down and open, from tail (n.) + gate (n.). Extended by 1950 to hatchback door on an automobile. The verb meaning "to drive too close behind another vehicle" is from 1951; tailgate party "party or picnic at the open tail-gate of a parked car" is attested from 1961.
The buttocks; ass (1940s+)