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Let sleeping dogs lie.
Literal
Sleeping dog [topic-は] let-sleep [imperative-おけ].
眠らせる is the causative of 眠る ('let sleep'). ~ておく ('leave in a state, keep') in the plain imperative おけ — 'keep it so.' Combined: 'keep [the dog] in the state of being let sleep.' A Japanese rendering of the English proverb, using the causative to maintain the sleep state.