The Collaborative International Dictionary of English v.0.48:

Swap \Swap\, v. t. [imp. & p. p. Swapped; p. pr. & vb. n.
   Swapping.] [OE. swappen to strike; cf. E. to strike a
   bargain; perh. akin to E. sweep. Cf. Swap a blow, Swap,
   v. i.] [Written also swop.]
   1. To strike; -- with off. [Obs. or Prov. Eng.] "Swap off his
      head!" --Chaucer.
      [1913 Webster]

   2. To exchange (usually two things of the same kind); to
      swop. [Colloq.] --Miss Edgeworth.
      [1913 Webster]