Green’s Dictionary of Slang

1. an exciting and successful person, thing or event.

1946		Mezzrow & Wolfe Really the Blues 117: Josh Billings [...] came up with a tricky homemade muffler that was a gangbuster.
1997		C. Hiaasen Lucky You 201: It was a gangbuster of a visitation.

2. a thug, a hoodlum; also attrib.

1939		R. Chandler ‘Trouble Is My Business’ in Spanish Blood (1946) 198: Maybe you better skip the gang-buster stuff and tell me what’s stuck in your nose.
1966		(con. 1958) R. Farina Been Down So Long (1972) 202: Three helmeted soldiers rode the roof with automatic weapons [...] ‘Gangbusters,’ said Heff.
1975		E. Torres Carlito’s Way 87: My boys was all bad gangbusters too.
1984		W.J. Caunitz One Police Plaza 121: ‘And if they need help?’ David asked. ‘Then we make like fucking Gang Busters,’ Malone said.