The truck was decades old, and it lacked a tailgate so the people in back were crammed together to avoid falling out.
Except for the unhappy expressions on their faces, they looked like they had settled in for a tailgate party.
A number of adults at the tailgate Party are wearing orange T-shirts printed with the words “See You at the Pole Event Staff.”
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+)