Green’s Dictionary of Slang

tinny n.2

[? tin n. (1)]

(US Und.) a thief.

[US]‘Goat’ Laven Rough Stuff 85: I met a pretty wide guy and fairly well dressed. He looked at me and I looked at him, and we both understood one another in a glance. He knew I was a ‘tinny’ (a thief) and I knew he was the same.