Green’s Dictionary of Slang

(US teen) depressing; depressed.

1972		(ref. to late 1960s) B. Rodgers Queens’ Vernacular 61: depress-o (kwn LV, late ’60s) despondent, feeling low, dejected ‘Don’t feel depress-o–you’re not worth it!’.
1976		R. Price Blood Brothers 125: Miss Guardino, I can’t hack it [...] It’s too depresso up there.