Green’s Dictionary of Slang

(Aus./US) a greeting; an inquiry as to what is happening.

1903		‘Hugh McHugh’ Out for the Coin 83: I catch your words, Murf, but the meaning is away to the fritz [...] what’s doing?
1915		Wodehouse Psmith Journalist (1993) 309: ‘Well,’ he said, ‘what’s doin?’.
1927		E. Philpotts Portreeve 278: Whereupon the ancient arose with a start and then groaned, for his action had provoked a stab of rheumatism. ‘What’s doing? — What’s doing?’.
1936		Asch & Muir War Goes On 31: ‘What’s doing?’ ‘I don’t need to tell you what’s doing, for nobody’s likely to know as much about it as yourself,’ returned the young man.
1941		Howard ‘Heat’ in Mann Coast to Coast 124: ‘O.K., Tony,’ said Joe. ‘What’s doing?’.
1968		J. Alard He who Shoots Last 5: Wots been doin’ wif you Kenny boy?
1984		J. McNamara First Directive (1985) 173: What’s doing?
2001		Sydney Morning Herald (Aus.) 6 Jan. n.p.: So here’s a tentative guide to Sydney teenspeak: [...] Wozzup? Wosdoin’? (Hello, how are you, what is happening, what are you doing?).