Douglas Harper's Etymology Dictionary
tail-gate
1868, back panel on a wagon, hinged to swing down and open, from tail (n.1) + gate (n.). Extended by 1950 to hatchback door on an automobile. The verb (also tailgate) meaning "to drive too close behind another vehicle" is from 1951 ("Truck drivers know the practice of following too close as tail-gating" - "Popular Science," Jan. 1952); as an adjective, in reference to the open tail-gate of a parked car as a setting for a party or picnic, from 1958. Related: Tail-gating.