Green’s Dictionary of Slang

lag v.3

[? fig. use of lag v.1 (1), i.e. ‘pissing around’/‘piss on’]

1. [1950s+] to talk repetitively and tediously.

2. [1950s+] (Aus. Und.) to inform.

3. [1990s+] (also lag off) to shirk one’s duties.

4. [2000s] (S.Afr.) to laugh.

In phrases

take lag (v.)

[1950s] (W.I./UK black) to criticize, to attack verbally.