Green’s Dictionary of Slang

deadly adj.

[on bad = good model]

(orig. US black/campus) excellent, first-rate.

[US]W.M. Raine Wyoming (1908) 234: Chalkeye was deadly on the shoot, and was ready for it at the drop of a hat.
[US]Life 27 Jan. 78: An orchestra that [...] sends [...] is called deadly [W&F].
[Ire]P. Boyle All Looks Yellow to the Jaundiced Eye 118: His face was wreathed in a delighted self-satisfied grin [...] Leaning out towards me, he whispered: ‘Deadly!’.
[US]G. Tate ‘Public Enemy’ in Flyboy in the Buttermilk (1992) 127: Got to say, though, the thrash is deadly.
[Ire]R. Doyle Snapper 99: Thanks. Rapid. Ah deadly.
[Ire]P. Howard Curious Incident of the Dog in the Nightdress 235: ‘It’s, like, a waterpork and shit?’ and Ronan’s like, ‘Fooken deadly.’.
[Ire]L. McInerney Rules of Revelation 298: ‘I think we need to do a gig, actually.’ ‘At some point that’d be deadly,’ Joseph said.