boo!; peek-a-boo!
Used when suddenly showing one's face to a baby or small child, often while hiding and then revealing oneself. The sound is typically drawn out.
I said "boo!" and made the baby laugh.
Variant with a long vowel mark, often used in manga or casual writing to emphasize the drawn-out sound.
A sound-symbolic interjection; the exact origin is uncertain, but it is widely used in baby talk and peek-a-boo games.