As a verb, it means to say something aloud. As an adjective, it means complete or total, often used for emphasis.
Verb: from Old English 'ūterian' (to put out, speak). Adjective: from Old English 'ūtera' (outer, farthest), later shifting to mean 'complete' or 'absolute'.
The adjective utter once meant 'outermost', then slid into 'to the furthest degree', and finally 'absolute'. So 'utter nonsense' is nonsense pushed all the way to the edge.
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