Green’s Dictionary of Slang

a layabout, a ne’er-do-well, a lazy person.

1910		Sporting Times 1 Jan. 1/5: Two Labour lags who lounged hard by laughed so immoderately that they almost let the pigeon go unstripped.
1936		(con. 1830s–60s) ‘Miles Franklin’ All That Swagger 105: He longed for Molly to marry into one of those families which had snubbed him as a lag, a terrier from the bogs of Ireland.
1937		J. Curtis You’re in the Racket, Too 97: Plenty of old lags lay about on Trafalgar Square benches and sat around kip-house fires.
1996		C.I. Macafee Concise Ulster Dict.
1999		Guardian Weekend 19 June 22: A resurrection that turned five wasted lags into the pre-eminent corporate rock cartoon of our time.