Green’s Dictionary of Slang

a general intensifier, great.

1870		A. Whitney We Girls xci n.p.: ‘It’s a dumb shame!’ said Aunt Trixie [DA].
1911		H. Quick Yellowstone Nights 104: A fellow makes a dumb fool of himself such times [DA].
1923		H.C. Witwer Fighting Blood 29: Its only by dumb luck that I don’t run the flivver right into the hedge.
1929		E. Dahlberg Bottom Dogs 228: Why the dumb bitchin’ hell didn’t yer wake me.