Green’s Dictionary of Slang

goodbye, esp. in communities influenced by California’s post-hippie n.² (3) era ‘new therapies’.

1961		M. Terry Old Liberty (1962) 61: No strain, boy. Stay loose.
1977		C. McFadden Serial 14: And remember [...] stay loose.
1981		J. Ellroy Brown’s Requiem 235: Stay loose, Hot Rod. I’m blowing town for a while.