taking a cut from a thief's plunder; vile deed
Idiomatic expression describing a despicable act of stealing from a thief or profiting from ill-gotten gains. Often used figuratively to condemn morally corrupt behavior.
You must never do something as vile as taking a cut from a thief's plunder.
Means 'to take a cut' or 'to skim off the top', often in a business or illegal context, but without the specific 'thief' element of 盗人の上前を取る.
The expression combines 盗人 (thief), 上前 (a cut or share taken from a transaction), and 取る (to take). The exact historical origin is uncertain, but it likely arose as a vivid metaphor for an especially low act.