Green’s Dictionary of Slang

a feat, an achievement, a performance; thus do/set dags, to do something that the other person cannot do, to show off.

1859		Hotten Dict. of Modern Sl. etc. 29: DAGS, feat or performance. ‘I’ll do your dags,’ I will do something that you cannot do.
1864, 1867, 1870		Hotten Sl. Dict. [as cit. 1859].
1879		N&Q Ser. 5 XII 15 Aug. 128: I’ll do you (or your) dags. An expression [...] meaning, ‘I’ll do something that you cannot do’ [F&H].
1886		Fun n.p.: He was very fond of what, in schoolboy days, we used to call doing dags [F&H].
1905		Marvel III:60 31: Here’s the spear. Do your dags on it?
1929		J.B. Booth London Town 156: Doing one another’s ‘dags’ for the entertainment of the amiable stranger.
1943		R. Llewellyn None But the Lonely Heart 213: Everybody was arse to arse [...] and what with dodging beefy blokes trying to do their dags, and ducking tarts elbows sharp as nails digging your ears out.