Green’s Dictionary of Slang

aggressiveness.

1977		(con. 1961) P. Caputo Rumor of War 28: The platoon sergeant [...] Six feet three inches and two hundred and twenty pounds of pure mean.
1988		D. Woodrell Muscle for the Wing 135: Brazen dash, rough talk, and an ounce or two of mean were clearly required.
2000		F.X. Toole Rope Burns 174: He liked to see some mean in a fighter.