late 14c., piken "look quickly and slyly," of unknown origin. The words peek, keek, and peep all were used with more or less the same meaning 14c.-15c.; perhaps the ultimate source was M.Du. kieken. The noun meaning "a peek, glance" is attested from 1844. Phrase peek-a-boo as a children;s game is attested from 1599; as an adj. meaning "see-through" it dates from 1895.