Green’s Dictionary of Slang

(Aus./US black) cruel, coldhearted.

1911		A. Wright Gamblers’ Gold (1931) 109: I ain’t goin’ to be dog enough to send you along now.
1985		T.R. Houser Central Sl. 19: dog, that’s Mean, cruel coldblooded. ‘Police goin’a tow da boy’s ride; that’s dog’.
2010		L. Redhead Thrill City [ebook] Then there’s your boyfriend. Don’t think we can’t get to him just ’cause he’s a dog cop.
2015		N. Cummins Tales of the Honey Badger [ebook] In short, thong theft is a dog act.