The Collaborative International Dictionary of English v.0.48:

Bilk \Bilk\, v. t. [imp. & p. p. Bilked; p. pr. & vb. n.
   Bilking.] [Origin unknown. Cf. Balk.]
   To frustrate or disappoint; to deceive or defraud, by
   nonfulfillment of engagement; to leave in the lurch; to give
   the slip to; as, to bilk a creditor. --Thackeray.
   [1913 Webster]