Green’s Dictionary of Slang

slag (off) v.

also slag off at
[slag n.1 ]

to criticize, to slander, to attack verbally.

[UK]L. Dunne Goodbye to The Hill (1966) 71: There was always somebody to hold a place for me in the queue. Usually when I crept in [...] some oul’ one behind would start slagging me for not taking my turn.
[UK]J. Mandelkau Buttons 75: He was doing a good job of bad mouthing and slagging me to a number of Angels.
[UK]G.F. Newman Villain’s Tale 91: [He] slagged off other drivers who got in his way.
[UK]A. Payne ‘Senior Citizen Caine’ in Minder [TV script] 58: Not that I want to slag off Arthur.
[UK] in R. Graef Living Dangerously 157: They used to slag me in front of her.
[Aus]M.B. ‘Chopper’ Read How to Shoot Friends 28: While I may slag off at most of them, girls like Tauree are the exception.
[Ire]P. Howard The Joy (2015) [ebook] Unless that’s [i.e. a love letter] code for something, he’s going to get slagged to death in the morning.
[US]Hip-Hop Connection Dec. 8: Verbal abuse from fans when we slag off their favourite rappers is bad enough.
[UK]J.J. Connolly Layer Cake 178: Freddie was givin it large, slagging everyone.
[UK]Guardian G2 4 Mar. 18: He started slagging off Sid Fearlittle, another con we knew.
[Aus]L. Redhead Thrill City [ebook] She was slagging me off to anyone who’d listen, telling them [...] I’d only got published because I was young and good-looking and I’d slept with the right people.
[UK]Independent 24 Jan. 37/1: Nobody Gagas, or Ke$has or Beyoncés or slags anyone off.
[UK]Guardian 31 Oct. [Internet] Fox News Slagging Off Radiohead.

In derivatives

slagging (n.) (also slagging off)

criticism, verbal attack.

[UK]T. Parker Frying-Pan 20: The Wing Meetings are shocking, all they are is slagging sessions.
[UK]S. Berkoff Decadence in Decadence and Other Plays (1985) 28: What a double choke to suffer slagging from the slut I poke.
[UK]M. Frayn Now You Know 152: She gave us a right slagging-off last night.
[UK]N. Cohn Yes We Have No 333: A lot of racial slagging on both sides.
[UK](con. 1980s) I. Welsh Skagboys 10: Andy n some other boys join in n ah jist take the slaggin cause it’s pointless arguing wi dozy auld fuckers.