means A small opening in a door or lock shaped to receive a key, through which the lock is operated — and through which a curious eye can spy.

from A plain English compound of "key" and "hole," the two oldest building blocks doing exactly what they say. "Key" comes from Old English "cǣg," a word for the device that opens a lock whose deeper roots are murky; "hole" from Old English "hol," a hollow or cavity. Joined together, they name the gap the key slips into — and English, ever practical, never bothered with anything fancier. The peeping sense ("keyhole view," "keyhole journalism") came later, once doors had locks worth spying through.