Green’s Dictionary of Slang

dab adj.²

bad.

1856		H. Mayhew Great World of London I 5: I’ve had a reg’lar troseno (bad sort) to-day. I’ve been doing b----y dab (bad) with my tol (lot, or stock).
1859, 1860		Hotten Dict. of Modern Sl. etc.
1867		Berks. Chron. 7 Dec. 7/1: The Language of Costers. We’ve a slang, sir, and it is only know to ourselves. It puzzles the Irish and bothers the Jews [...] A doogheno or dabheno for ‘it is a good or bad market’.
1877		J. Diprose London Life 43: [as cit. 1856].
1882		Sydney Sl. Dict. (2 edn) 11: Dab - Bad.
1922		Tamworth Herald 5 Aug. 3/5: ‘Dab brief’, slang for a cheque, is explained as back slang for ‘fear bad’.
1999		G. Seal Lingo 40: Larrikin backslang formations included occabot for tobacco and dab for bad.