Green’s Dictionary of Slang

1. an all-purpose intensifier, e.g. serious drinking.

1915		Van Loan ‘Sporting Doctor’ in Taking the Count 20: Betting them as much money as they would take [...] put a serious crimp into his bank account.
1929		M. Prenner ‘Sl. Terms for Money’ in AS IV:5 357: Recently we have begun to hear (or at least, to read, in such writings as those of H.C. Witwer) of important or serious money or of heavy sugar.
1949		Monteleone Criminal Sl. (rev. edn) 203: serious money A large amount of money.
1967		M. Frayn Towards the End of Morning (2000) 177: It’s a week off from the stinking office, with nothing to do but [...] get some serious drinking done.
1982		Barr & York Sloane Ranger Hbk 159: serious, seriously adj & adv. The Sloane way of exaggeration. ‘He’s seriously rich.’.
1989		C. Hiaasen Skin Tight 160: She beat you and the anchorman out of some serious dough.
1996		‘Iceberg Slim’ Trick Baby [blurb] Robert Beck, who used Iceberg Slim as his moniker, was a major league pimp who enjoyed serious success in Chicago.
2001		N. Barlay Hooky Gear 123: Plot your moves from there. Serious moves.
2021		Breen & Conlon Hitmen 10: Heckler & Koch MP5 sub-machine guns and Sig Sauer P226 handguns [...] sawn-off and automatic shotguns [...] a replica AK-47. This was some serious firepower.

2. (US black) excellent, first-rate.

1982		W. Safire What’s The Good Word? 79: Your mother’s cooking may be baad — i.e., very good — but your grandmother’s cooking is serious.
1992		K. Lette Llama Parlour 154: They are a serious set of jugs.
2000		Guardian Guide 1–6 Jan. 18: Thanks to this serious ill shit bit of kit my dad bought back from his business trip to Bangkok.