Past tense of coo; made soft murmuring sounds like a dove or pigeon, or spoke gently and tenderly.
From Old English cu, imitative origin from the sound doves make. The word is onomatopoetic—it literally imitates the sound, and has meant 'make dove sounds' for over a thousand years.
Coo is one of the oldest onomatopoetic words in English, unchanged for a millennium because the sound doves make hasn't changed—it's a direct line from ancient Anglo-Saxon language to how we describe cooing today.
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