The Collaborative International Dictionary of English v.0.48:

Boy scout \Boy scout\
   Orig., a member of the "Boy Scouts," an organization of boys
   founded in 1908, by Sir R. S. S. Baden-Powell, to promote
   good citizenship by creating in them a spirit of civic duty
   and of usefulness to others, by stimulating their interest in
   wholesome mental, moral, industrial, and physical activities,
   etc. Hence, a member of any of the other similar
   organizations, which are now worldwide. In "The Boy Scouts of
   America" the local councils are generally under a scout
   commissioner, under whose supervision are scout masters, each
   in charge of a troop of two or more patrols of eight scouts
   each, who are of three classes, tenderfoot, second-class
   scout, and first-class scout.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

Boy Scout
    n 1: a boy who is a member of the Boy Scouts
    2: a man who is considered naive