The Collaborative International Dictionary of English v.0.48:

Bonce \Bonce\, n. [Etymol. unknown.]
   A boy's game played with large marbles.
   [1913 Webster]

WordNet (r) 3.0 (2006):

bonce
    n 1: informal terms for a human head [syn: attic, bean,
         bonce, noodle, noggin, dome]