Green’s Dictionary of Slang

God-awful adv.

appallingly.

1936		R. Chandler ‘Guns At Cyrano’s’ in Red Wind (1946) 215: Crummy. Godawful crummy.
1955		Boston Globe (MA) Fiction Mag. 18 Sept. 2/1: ‘Gawdawful hot, ain’t it?’.
1977		L. Heinemann Close Quarters (1987) 318: Let’s go, I’m god-awful tired from the trip, let’s go.