Think of a dove sitting on a wire, seen from behind. Its tail is narrow where it leaves the body and spreads out wide at the end. Now cut a piece of wood into that shape and push it into a hole of the same shape. Because it grows wider as it goes in, it cannot slide back out the way it came. Everything the word does grows from that one fact: two shapes made for each other, and a hold that needs no nails.
可以想成一隻鴿子背對著你停在電線上。牠的尾巴靠近身體的地方窄，末端張開變寬。現在把一塊木頭切成那個形狀，再推進同樣形狀的凹槽裡。因為它愈往裡愈寬，所以沒辦法照原路滑出來。這個字後來的每一種用法，都從這一件事長出來：兩個形狀是為彼此做的，湊上以後不用釘子也拔不開。
From the word’s history
The noun is dove plus tail: the joint is named after the shape of a dove's tail. The verb was formed from the noun afterwards.
名詞就是 dove（鴿子）加 tail（尾巴），直接用形狀命名。動詞是後來從名詞長出來的，所以先有那個榫，才有「把木頭榫起來」這個動作，再往後才有抽象的「剛好吻合」。
Wiktionary, "dovetail"
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