Green’s Dictionary of Slang

1. to urinate.

1741		Canting Academy, or the Pedlar’s-French Dict. 118: To Piss To Lag.
1812		Vaux Vocab. of the Flash Lang.
1846		Swell’s Night Guide 50: ‘Your doss gorger cracked a wid about you to me, and said she must give you the shoot.’ ‘Shoot! what for?’ roared poor Fuzzy [...] ‘Why because you made a dunniken of your cupboard, and used to lag in the coffee pot’. [Ibid.] 73: There’s not a lagging gage on the estate, and every padder shoots his lagging slum in the chimbley corner.
1859, 1860		Hotten Dict. of Modern Sl. etc.
1864, 1867, 1870		Hotten Sl. Dict.
2023		R. Milward Man-Eating Typewriter 306: While I lagged, I fancied it might be proper to avert my opals.

2. (UK Und.) to water down.

1812

Vaux Vocab. of the Flash Lang. in McLachlan (1964) 249: To lag spirits, wine, &c., is to adulterate them with water.

3. to drink.

2000		J.J. Connolly Layer Cake 256: Guys with great big bandages round their heads from [...] falling over lagging and landing on the bonce.
2011		J.J. Connolly Viva La Madness 294: They’re a load of lagging bioats and smackheads.