Green’s Dictionary of Slang

1. calm, unperturbed, in control.

1931		D. Runyon ‘Romance in the Roaring Forties’ in Runyon on Broadway (1954) 34: It is some time before I can get him cooled out enough to explain.
1972		E. Grogan Ringolevio 222: Kenny kept himself cooled out by smoking reefer [...] with a few black dudes.

2. under the influence of drugs, usu. narcotics.

1994

N. McCall Makes Me Wanna Holler (1995) 375: They too busy noddin’ out. They too doped up and cooled out to tear up anything.