The Collaborative International Dictionary of English v.0.48:

Whippersnapper \Whip"per*snap`per\, n.
   A diminutive, insignificant, or presumptuous person.
   [Colloq.] "Little whippersnappers like you." --T. Hughes.
   [1913 Webster]

WordNet (r) 3.0 (2006):

whippersnapper
    n 1: someone who is unimportant but cheeky and presumptuous
         [syn: whippersnapper, jackanapes, lightweight]