Green’s Dictionary of Slang

important, urgent, impressive etc.

1958		H. Ellison Web of the City (1983) 25: Lemme alone, and I let your big deal Prez alone.
1970		D. Ponicsan Last Detail 3: It’s something really big-deal I think.
1979		D. Maitland Breaking Out 55: I suppose you’re a big-deal crim.
1981		J. Wambaugh Glitter Dome (1982) 46: It wasn’t as though it was some big-deal homicide anyway.
1993		M.B. ‘Chopper’ Read How to Shoot Friends 53: The big deal greyhound blew his monkey muscle at the time trials.
2019		N. Walker Cherry 8: And you might be crazy, and you might have a gun, but even then it’s usually no big deal.