Peek-a-boo basically means “peeking out”.

It probably started when adults wanted to entertain babies.

Hiding your eyes, then peeking out from behind your hands and saying “boo” would startle and entertain an infant.

The “Peek” part was the hiding, and the “boo” was the coming out. And it evolved to “peek-a-boo”.

It is now often used as a term for innocent fun.

And it has since been used to describe items that let things “peek” out, such as a peek-a-boo bra.

usually the adult is hiding and would peek out from behind a chair/door/cupboard and boo the child to make him/her laugh.

there’s no such thing as “peak a booh” or “peak a boo.”