Green’s Dictionary of Slang

lag v.3

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

1. to talk repetitively and tediously.

[UK]‘Charles Raven’ Und. Nights 166: He certainly was one to lag was Andy Andrews. He used to talk to me by the hour about his ambitions.
[UK]J.J. Connolly Layer Cake 99: If you get captured by Mickey on a bad night, when he’s had about ten pints with chasers, he’s lagging, all Queen and Country, up the Union jack, he’s a right pest.

2. (Aus. Und.) to inform.

[UK]J. Franklyn Cockney 162: Should there be black eyes and the like to account for, it is a strong point of honour to let the method of their acquisition remain a mystery to interested adults. To ‘lag’ in this manner is to invite ostracism or worse.
[Aus]R. Aven-Bray Ridgey-Didge Oz Jack Lang 35: Lag Inform.

3. (also lag off) to shirk one’s duties.

[US]S. King Dolores Claiborne 17: And if you lagged off, she was apt to catch you. She didn’t watch every time you beat the welcome mat, but lots of times she would.
[US]L. Pettiway Workin’ It 200: I started lagging in school.

4. (S.Afr.) to laugh.

[SA]A. Lovejoy Acid Alex 162: I was going to lag it off.

In phrases

take lag (v.)

(W.I./UK black) to criticize, to attack verbally.

[UK]S. Selvon Lonely Londoners 74: When Ma not there Tolroy used to take lag on Tanty left and right for coming to London.