The Collaborative International Dictionary of English v.0.48:

Whipperin \Whip"per*in`\, n.
   [1913 Webster]
   1. A huntsman who keeps the hounds from wandering, and whips
      them in, if necessary, to the of chase.
      [1913 Webster]

   2. Hence, one who enforces the discipline of a party, and
      urges the attendance and support of the members on all
      necessary occasions.
      [1913 Webster]

WordNet (r) 3.0 (2006):

whipper-in
    n 1: huntsman's assistant in managing the hounds