Green’s Dictionary of Slang

(US campus) to feel depressed.

1985		Eble Campus Sl. Oct. 2: bum – to be down or depressed [...] ‘I’m bumming over that call I got last night.’.
1993		M. Myers et al. Wayne’s World II [film script] Wayne, are you bumming?
1997–2001		Online Sl. Dict. 🌐 bum v 1. to be depressed. (‘Are you bumming about her again?’).
2020		(con. 1991-94) W. Boyle City of Margins 55: He’s still bummed that she took pity on him like that.