Green’s Dictionary of Slang

(US und.) a villain who disparages his peers behind their backs.

1909		‘The Lang. of Crooks’ in Wash. Post 20 June 4/2: [paraphrasing J. Sullivan] A criminal who slanders another among his friends is a Crowley’s anvil.
1935		D. Maurer ‘Lingo of the Good-People’ in AS 10:1 n.p.: CROWLEY'S ANVIL. 1. A malicious gossip; one who talks too much. (Obs.) .
1949		Monteleone Criminal Sl. (rev. edn).