Green’s Dictionary of Slang

(UK black) a general term of congratulation, an admirable person.

1997		C. Newland Scholar 171: Hear what, I don’t care, my man’s a dapper for dat move!
2003		Dizzee Rascal ‘Wot U On?’ 🎵 You could be a dapper, you could be a don.
2017		theculturetrip.com ‘Guide to London Slang 10 Jan. 🌐 Dapper – someone who looks fancy or smart.