Picture something already standing — a wall, a house, a roof — and someone working underneath it, setting solid pieces in place so the weight above has something to rest on. The thing on top does not move and does not change. The work is all below, out of sight, and it is what decides whether the top stays up.
可以想成一面牆或一間房子已經站在那裡，有人鑽到它下面，把一塊塊硬東西墊上去，讓上面的重量有地方靠。上面那個東西沒有動，也沒有變樣子。所有的工都發生在下面，而且看不到。但它決定了上面撐不撐得住。underpin 每次用，指的都是這個「墊在底下、看不見、卻是關鍵」的位置。
From the word’s history
Wiktionary gives underpin as under- plus pin, from Old English pinn, a peg or short post. The word is literally 'to put pegs underneath', which is still exactly what the building sense describes.
Wiktionary 記載 underpin 由 under- 加上 pin 組成，pin 來自古英語 pinn，指釘子或短柱。字面就是「在下面打上短柱」。今天的工程用法幾乎還是字面本身，抽象的用法則是從這個畫面走出去的。
Wiktionary etymology tree supplied in evidence packet (kaikki:3f6f69a190036ae4:881387617:16487)
登入就可以看每一支怎麼連回核心畫面。