Green’s Dictionary of Slang

peak adj.

unpleasant, depressing; thus as n. bad news.

2012		Guardian 3 May 🌐 ‘I don’t understand why they are showing a film like that here, it’s peak,’ says one of the young men, explaining [...] that ‘peak’ means ‘a bit terrible’.
2014		Quora 🌐 One of the most widely used and confusing (to outsiders) slang words is peak, basically the meaning the superlative form of bad, similar to 'the worst' but often used ironically.
2014		Wiley ‘Busy’ 🎵 I was like ‘babe, this week's schedule might be peak’ / Gotta get the album mixed.
2016		Skepta ‘Lyrics’ 🎵 Yeah, you got murked last week Couldn’t even get a rewind, that’s peak.
2018-20		T. Thorne (ed.) ‘Drill Slang Glossary’ at Forensic Linguistic Databank 🌐 Peak - bad news.
2020		G. Krauze Who They Was 5: This is so peak, it can’t end like this, it can’t.

In derivatives

peaking (adj.)

(N.Z. prison) excellent, first-rate, wonderful.

2001

D. Looser Boobslang [U. Canterbury D.Phil. thesis] 137/2: peaking adj. excellent, fantastic, great.