Green’s Dictionary of Slang

lag n.1

[? OF laige/laigue, water]

1. [mid-16C–18C] a bundle of clothes; always as lag of duds n.

2. [late 16C–19C] weak liquor or wine.

3. [17C–19C] urine.

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)