Past tense of couch; also means expressed in particular words or phrased in a certain way.
From Old French couchier (to lie down), the past tense form; also developed the figurative sense of 'expressing' ideas that are 'laid down' in words.
When you 'couch' an argument in careful language, you're choosing exactly which words to 'lay down' to present your idea—it's the same root as a piece of furniture you lie on, which shows how metaphorically rich English can be.
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