浴缸、水槽底下的那個洞
這一家就是核心本身：看得見、摸得到的那個洞口。
noun · B2
The hole at the bottom of a bath, sink or shower that the water runs out through, and that you can close with a plug to keep the water in.
排水孔
核心畫面
You can think of a plughole as the one small round gap at the bottom of a sink or bath through which everything leaves. Put the plug in and the water stays. Take it out and the water turns once, drops, and is gone. Whatever follows it down is not coming back.
plughole 的核心畫面可以想成水槽或浴缸底部那個小圓洞：整槽的水只有這一個出口。塞子放著，水就留住；塞子一拔，…更多收起
plughole 的核心畫面可以想成水槽或浴缸底部那個小圓洞：整槽的水只有這一個出口。塞子放著，水就留住；塞子一拔，水轉一圈就掉下去了。跟著水下去的東西，通常也撿不回來。這個「轉一圈就沒了、而且拿不回來」的感覺，就是後面那些比喻用法的來源。From the word’s history
The word is simply plug plus hole, so the literal sense and the word-parts sit on top of each other: the hole a plug goes into.
這個字就是 plug（塞子）加 hole（洞）。字面和意思幾乎重疊：塞子塞的那個洞。所以核心畫面不是推想出來的，它就寫在字面上。
Wiktionary extract supplied in packet (source_ref kaikki:3f6f69a190036ae4:1677937845:2920): From plug + hole.
Two branches
down the plughole：轉一圈就沒了
水進了那個洞就回不來。英文把這個畫面整句借走，用來講「白花力氣」和「店收掉」：東西不是被誰拿走，是自己轉一圈就不見了。兩個意思用的是同一句話 go down the plughole，靠主詞分辨。
noun · B2
If work, money, time or effort goes down the plughole, it is completely wasted and there is nothing left to show for it.
付諸流水