Green’s Dictionary of Slang

anything excessive, esp. a great lie.

1672		J. Eachard Hobbs’s State National Letter 35 n.p.: It may be convenient for you to call this [...] a flam, a whisker, a caprice, a piece of fright, malice> calumny and spleen.
1687		C. Sedley Bellamira I i: These are two Whiskers!
c.1698		B.E. Dict. Canting Crew n.p.: Whisker a great Lie.
1725		New Canting Dict. [as cit. c.1698].
1737, 1759, 1760, 1776		Bailey Universal Etym. Eng. Dict. [as cit. c.1698].
c.1750–1882		B.M. Carew Life and Adventures.
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue n.p.: Whisker, a great lye.
1809		G. Andrewes Dict. Sl. and Cant.
1811		Lex. Balatronicum [as cit. 1785].
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.
1828		Sussex Advertiser 14 Apr. 4/3: ‘That’s a “whisker”,’ says Phil.
1835		G. Kent Modern Flash Dict. 35: Whisker – a bouncing lie.
1848		Flash Dict. in Sinks of London Laid Open [as cit. 1835].
1859		Matsell Vocabulum.
1864, 1867, 1870		Hotten Sl. Dict. 269: When an improbable story is told, the remark is, ‘the mother of that was a whisker,’ meaning it is a lie.
1895		Crowe Aus. Sl. Dict. 95: Whisker, an enormous lie.