Green’s Dictionary of Slang

1. to relax, to remain calm.

1957		Laurents & Sondheim West Side Story I vi: Stay loose, boy! / Breeze it, buzz it, easy does it.
1965		H. Rhodes Chosen Few (1966) 63: Stay loose, baby. If these gray boys start anything, there are enough of us around t’finish it.
1977		C. McFadden Serial 64: You gotta stay loose, you know?
1999		Argus leader (Sioux Falls, SD) 6 May 15/2: ‘I just tried to relax and stay loose’.
2005		Hartford Courant (CT) 1 Feb. C04/1: You stay loose like a boxer stays loose.
2016		Morn. Call (Allentown, PA) 17 July C4/3: Pitrchers throw, but it’s just to stay loose, stay sharp.

2. to keep an open mind.

1983

S. King Christine 131: I’m just telling you to stay loose, that’s all.