Green’s Dictionary of Slang

calm, relaxed, ’cool’.

1986		C. Stroud Close Pursuit (1988) 119: He was supposed to be able to do this stuff [i.e. street crime]. He was chilled-out. He could kill!
1999		Guardian Rev. 25 June 20: We’re just average chilled-out guys.
2000		Guardian Rev. 11 Feb. 11: Listen to the music of the most chilled-out man who ever lived.
2008		IOL News (Western Cape) 4 Mar. 🌐 Troy’s raucous long-suffering mother and his chilled-out, beanie-wearing mate.