Soft, bubbling sounds like water flowing or a baby making happy sounds in its throat.
From imitative origins, possibly related to 'gorge' or Old French 'gorge' (throat). The word is onomatopoetic, designed to sound like the thing it describes.
Babies' gurgles are actually how they're learning to control their vocal cords—when a baby gurgles, they're practicing every sound that will eventually become words, which is why parents instinctively respond to them!
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