Green’s Dictionary of Slang

(US) at an angle or slanted.

1931		D. Runyon ‘Tobias the Terrible’ in Runyon on Broadway (1954) 105: He has a brown hat sitting jack-deuce on his noggin.
1957		W. Coburn Stirrup High 14: I had my hat on at a jack-deuce slant and wished I had on the rest of my cowboy outfit.
1961		R.F. Adams Old-Time Cowhand (1989) 83: He learned to wear this hat at jes’ the right jack-deuce angle over his off eye.