Think of standing in water or mud and looking down: it comes up to your knees. Your own body is the ruler, and that is already enough to slow every step you take.
可以想成自己站在水裡或泥裡低頭看：它淹到膝蓋。這裡用的尺是自己的身體，而且光是這個深度，就足以讓每一步都變慢。
From the word’s history
Wiktionary traces knee-deep to Middle English kne-depe, which is simply knee plus deep.
來源資料把 knee-deep 追到中古英文的 kne-depe，就是 knee（膝蓋）加 deep（深）兩個字拼起來。
Wiktionary entry for knee-deep (kaikki extract, source_ref kaikki:3f6f69a190036ae4:1147710316:5667)
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