Green’s Dictionary of Slang

1. very much so, exceedingly.

1910		H. Hapgood Types From City Streets 38: A junk is a down bad man, real bad.
1915		E. Poole Harbor (1919) 263: You can be sure of that, I’ve got him down cold.

2. (orig. US black) to the limit.

1928		R. Fisher Walls of Jericho 299: Drunk down: the nadir of inebriation.
1942		Amer. Mercury (July) 94: Draped down: dressed in the height of Harlem fashion; also togged down .
1978		S. King Stand (1990) 635: There were two dancers in silhouette and the words BOOGIE DOWN! on its hot pink cover.