Picture the back end of a small truck. Tail is the end that trails behind everything else; gate is the board that closes it. Let that board down and it lies flat: you can load boxes on it, sit on it, or put food on it. Stand right up against it and you are on the vehicle's tail, with no room in between. Nearly every use of tailgate keeps one of those two pictures — the board at the very back, or the spot pressed up close behind something.
From the word’s history
Wiktionary gives the word as tail + gate (compare liftgate) — literally the gate at the tail end. That reading sits directly under the modern uses: the board at the back, the party held at it, and the position right behind something.
Wiktionary 把這個字拆成 tail（尾端）+ gate（門、閘板），字面就是「尾端那道門」。這個拆法正好對得上今天的用法：車尾那片門板、在門板旁邊辦的聚會，以及緊跟在別人後面的位置。
Wiktionary, entry 'tailgate' (kaikki extract, source_ref kaikki:3f6f69a190036ae4:1117039291): 'From tail + gate. Compare liftgate.'
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