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]

The Collaborative International Dictionary of English v.0.48:

Swop \Swop\, v. & n.
   Same as Swap. --Dryden.
   [1913 Webster]

WordNet (r) 3.0 (2006):

swop
    n 1: an equal exchange; "we had no money so we had to live by
         barter" [syn: barter, swap, swop, trade]
    v 1: exchange or give (something) in exchange for [syn: trade,
         swap, swop, switch]