Green’s Dictionary of Slang

lag n.1

[? OF laige/laigue, water]

1. a bundle of clothes; always as lag of duds n.

2. weak liquor or wine.

[UK]Partridge DSUE (8th edn) 662/2: late C.16–19.

3. urine.

[UK]Rowlands Martin Mark-all E3: Lagge, water or pisse .

4. see lage n.

In phrases

take a lag in (v.)

(W.I.) in fig. use, to abuse, lit to urinate on, thus cognate with piss on v. (1)

[WI]S. Selvon Moses Ascending (1984) 45: When the communication media tired lambasting the Blacks and the Paddies, they take a lag in Paki arse.