Green’s Dictionary of Slang

(orig. US) an imper. excl., relax! enjoy yourself! don’t worry!

1955		S. Stern Rebel Without a Cause [film script] Hang loose, boy. I’m warning you [HDAS].
1963		J. Blake letter 8 Sept. in Joint (1972) 223: So hang loose, old friend.
1977		B. Davidson Collura (1978) 92: Hang loose, kid. We’ll take care of it.
1988		Beano Comic Library No. 12 4: Just hang loose, man.
1993		I. Welsh Trainspotting 44: Aw . . . Hang loose catboy.