Green’s Dictionary of Slang

1. to hit very hard.

[1900		Bulletin (Sydney) 10 Mar. 24/2: Nolan’s punches apparently carried most powder or they were luckier in landing on soft places, for the Cabbage-garden man was the first to fly signals of distress, and he continued flying them till a soul-case-shaking uppercut [...] sting caused his knees to sag and the whole lot of him to drop with a thud].
1942		C.R. Bond 20 Mar. in A Flying Tiger’s Diary (1984) 129: It took all the persuasion I ever had to keep Pappy from powdering me.

2. to kill, to murder.

1935		A.J. Pollock Und. Speaks n.p.: Sprinkle with powder, to put on the spot; to assassinate.
1988		D. Woodrell Muscle for the Wing 47: When a St. Bruno cop gets powdered [...] one or two or even three street studs turn up, shot dead tryin’ to escape.