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.
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.'
Four branches
車尾那一片門板
這一族就是核心畫面本身，其他意思都是從這片門板延伸出去的。
noun · B2
The door or board at the back of a truck, van or pickup that opens downwards and lies flat, so things can be loaded and unloaded easily. In British use it can also mean the rear door of a hatchback car, which lifts up instead.
車尾門／後擋板
門板放平，就成了桌子：車尾派對
把核心畫面裡「放平的門板」當成家具來用，整場聚會就用那片門板命名。
noun · B2
In the United States, a party in the car park outside a stadium before a game, where people cook and eat at the back of their cars while they wait for the gates to open.
車尾派對
貼在別人後面
核心畫面的另一半：不是那片門板本身，而是緊貼在門板後方、中間幾乎沒有空隙的那個位置。
verb · B2
To drive right behind another vehicle, so close that you would have no room to stop if the driver in front slowed down suddenly.
緊跟車尾／逼車
末端的那道閘門與通道
這一族回到 tail 最素樸的意思：不是車尾，而是一條路線或一股流動的末端；gate 則是守在那個末端的門或通道口。
noun · C1
The gate at the lower end of a lock on a canal or river, or in an irrigation system — the one on the downstream side, which is opened to let the water level fall so a boat can go out.
下游閘門