The Collaborative International Dictionary of English v.0.48:
Peekaboo \Peek"a*boo\, n. A child's game; bopeep. [1913 Webster]WordNet (r) 3.0 (2006):
peekaboo n 1: a game played with young children; you hide your face and suddenly reveal it as you say boo! [syn: peekaboo, bopeep]