Green’s Dictionary of Slang

a general intensifier, complete, utter, intense, e.g. steaming hangover.

1962		Listener 13 Dec. 1024/3: A cautionary tale concerning a real steaming nit of a British civilian.
1983		S. Gee Never in My Lifetime in Best Radio Plays (1984) 76: Listen, to hell with the whole steaming nightmare out there, who’s doing what and why.
2003		Observer 8 June 3: I roll in with a steaming hangover and no one notices.