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，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, "plughole"
登入就可以看每一支怎麼連回核心畫面。