Green’s Dictionary of Slang

a general intensifier, completely, very, usu. in negative.

1910		Sun. Times (Perth) 24 July 2nd sect. 10/5: As a rule these amateur events are won by steaming-hot favorites.
1942		C.R. Bond 21 May in A Flying Tiger’s Diary (1984) 178: I was steaming mad.
1991		C. Hiaasen Native Tongue 118: He was steaming mad.