To coo means to make the soft, gentle murmuring sound that pigeons and doves make. If you've ever heard a dove on a quiet morning, that low, soothing “coo-coo-coo” is the sound this word describes.

The word also describes the sweet, wordless sounds that babies make when they're happy and content. When a baby coos, they're making those adorable gurgling noises that aren't quite words yet but show they're pleased. Parents and grandparents often coo back at babies, using a gentle, musical voice.

People also use coo to describe speaking in an overly sweet or affectionate way, sometimes too much so. If someone coos over a cute puppy, they might use that high-pitched, gushing voice some people use around adorable animals.

The word captures that quality of gentleness and softness, whether it's coming from a bird on a branch, a happy infant, or someone being extra affectionate.