BOPEEP

peekaboo, bopeep

(noun) a game played with young children; you hide your face and suddenly reveal it as you say boo!

Source: WordNet® 3.1

Bo*peep", n. Etym: [Bo + peep.]

Definition: The act of looking out suddenly, as from behind a screen, so as to startle some one (as by children in play), or of looking out and drawing suddenly back, as if frightened. I for sorrow sung, That such a king should play bopeep, And go the fools among. Shak.

Source: Webster’s Unabridged Dictionary 1913 Edition