Green’s Dictionary of Slang

lag v.1

[lag n.1 /lage v.]

1. to urinate.

[UK]Canting Academy, or the Pedlar’s-French Dict. 118: To Piss To Lag.
[UK]Vaux Vocab. of the Flash Lang.
[UK]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.
[UK]Hotten Dict. of Modern Sl. etc.
[UK]Hotten Sl. Dict.

2. (UK Und.) to water down.

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

3. to drink.

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

In compounds

lagging gage (n.) [gage n.1 (2); despite Hotten, there exist no prior citations]

a chamberpot.

[UK]Swell’s Night Guide 50: Fuzzy stewed it [mutton] in a laggingage, and said it was bona mongary. [Ibid.] 73: There’s not a lagging gage on the estate, and every padder shoots his lagging slum in the chimbley corner.
[UK]Sl. Dict. 212: Lagging gage a chamber-pot. ― Ancient Cant.
[UK]Referee 8 Mar. n.p.: All this storm in a lagging-gage is very absurd [F&H].