Green’s Dictionary of Slang

something large of its type, e.g. a barefaced lie.

c.1700		W. Kennett MS. Lansd. 1033: Swapper, a great lie is called a swapper [OED].
1715		M. Davies Athenae Britannicae I Preface 36: After they have confess’d their swappers to the Jesuits or some of the Regulars [OED].
1818		W. Maginn in Blackwood’s Mag. IV 321: I’m a swapper, as every one knows, In my pumps six feet three inches high .