Green’s Dictionary of Slang

(US) silly, foolish, crazy; thus as n., a mad person.

1912		Bruner & Francis ‘A Short Word-List From Wyoming’ in DN III:7 550: dingy, foolish, not quite right in the mind; about the same as ‘batty’. ‘He’s dingy’.
1928		R.J. Tasker Grimhaven 32: You’re dingy. Why take a chance like that?
1936		(ref. to 1920s) L. Duncan Over the Wall 57: It’s stuff like this that makes a monkey stir-simple. We all get dingier’na pet coon inside these dumps.
1937		B. Dai Opium Addiction in Chicago 198: Dingy. An insane person.
1958		E. Gilbert Vice Trap 55: ‘Is he dingie?’ [...] ‘He’s serious.’.
1962		P. Crump Burn, Killer, Burn! 66: Eight big iron men all shot to hell and gone, ’cause an old dingy bastard wanted to impress a mother-feeling bishop.
1972		J. Wambaugh Blue Knight 287: He’s a little dingy. I think he’s gonna stab somebody before too long.
1986		J. Ellroy Silent Terror 198: [D]ingy, zorched, whacked-out and fried from 108 hours of continuous consciousness.
2005		B. Hare Urban Grimshaw 58: ‘What sort of book have you lost?’ ‘My Social Security book, dingy.’.