Hold the two halves of the word and the picture builds itself: pig, plus pen, which here is a small space closed in by a fence to keep animals in one place. So the picture is a low muddy square behind the farm buildings, a gate you have to check twice, a strong smell, and animals living close together. Everything else this word does is that square, moved somewhere new.
可以想成：把這個字拆成兩半，畫面自己就長出來了。pig 是豬；pen 在這裡不是筆，是用柵欄圍起來、把動物留在一個地方的小空間。所以畫面是農舍後面那一塊矮矮的泥地，一道要檢查兩次的門，很重的味道，動物擠在一起。這個字後面的每一種用法，都是把這一塊方方的地搬到別的地方去而已。
From the word’s history
The source records the word as a compound of pig + pen, so the modern uses all rest on that plain build.
來源把這個字記成 pig + pen 的複合字。所以今天的用法都疊在同一個很直白的組合上：關豬的那一圈。
Wiktionary, "pigpen"
登入就可以看每一支怎麼連回核心畫面。