Definitions

1. verb

to make a soft, low, humming or chirping sound, typically with the tongue against the roof of the mouth

“The birds cooed in the trees outside my window.”

2. interjection

an expression of soft, gentle, or soothing sound

“Oh, coo, that’s so sweet!”

3. noun

a low, humming or chirping sound, typically made by a bird

“The coo of the pigeon filled the air.”

4. onomatopoeia

a word that imitates the sound it describes

“The coo echoed through the forest.”