The Collaborative International Dictionary of English v.0.48:

Impawn \Im*pawn"\, v. t. [imp. & p. p. Impawned; p. pr. & vb.
   n. Impawning.] [Pref. im- + pawn: cf. Empawn.]
   To put in pawn; to pledge; to pawn. --Shak.
   [1913 Webster]