Green’s Dictionary of Slang

1. a gross, deliberate lie.

c.1698		B.E. Dict. Canting Crew n.p.: Clanker a swinging Lie.
1725		New Canting Dict.
1737, 1759, 1760, 1776		Bailey Universal Etym. Eng. Dict.
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue.
1811		Lex. Balatronicum.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.

2. (US tramp) a boxcar.

2011

H. Ellison Introduction in Pulling a Train’ [ebook] I’d manage to jump aboard an empty boxcar [...] they were called clankers.