Green’s Dictionary of Slang

bad adv.

as a general intensifier, very, extremely.

[US]Breckenridge News (Cloveport, KY) 23 Aug. 3/4: Hardinsburg has the railroad fever ‘bad’.
[US]A. Trumble Mott Street Poker Club 44: ‘Yo sabbe you hab ’em velly bad to-day’.
[US]E.A. Settle ‘From the Blackjacks’ in Botkin Folk-Say 247: George got down bad sick.
[UK]M. Anthony Green Days by River 52: She said ‘You enjoying it?’ ‘I enjoying it bad, girl.’.
[US]A. Young Snakes (1971) 107: Cat looked bad, man. I mean bad-bad, he was so messed up he could hardly walk.