Green’s Dictionary of Slang

a general intensifier: incredibly, remarkably, extremely.

1844		A.W. Kinglake Eothen 173: The joyous girls will suddenly, and screamingly, and all at once, explain.
1879		Mons. Merlin 21 Mar. 7/2: ‘I was at an awfully nice dinner party last night,’ says one. ‘You should see the new farce,’ says another, ‘it is screamingly funny’.
1885		Bulletin (Sydney) 11 Jul. 11/4: And this thought occurs to us, screamingly funny, / How different our language, behaviour, and views / From those of our ancestors, hard up for money, / Who did, like ourselves, little bills with the Jews!
1916		J. Lait ‘Charlie the Wolf’ Beef, Iron and Wine (1917) 32: A woman, dressed screamingly, painted gaudily, and carrying herself as no one who knew could mistake.
1932		D.L. Sayers Have His Carcase 435: What more natural than that Weldon, if questioned, should remember a number so screamingly funny as that? Oi, oi, oi! Highly humorous.
1947		N. Marsh Final Curtain (1958) 38: I don’t think your line of comedy with Barker is screamingly funny.