Green’s Dictionary of Slang

a strange and unaccountable story.

1720		Spiller in Anti-Theatre No. 13 38: Robinson Crusoe [...] has distinguished himself by many strange and unaccountable stories, which your smart fellows in conversation are pleased to call guns [OED].
1725		New Canting Dict. n.p.: gun [...] a Lye.
1737, 1759, 1760, 1776		Bailey Universal Etym. Eng. Dict. [as cit. 1725].