Green’s Dictionary of Slang

down v.2

[abbr. SE knock down]
(US)

1. [mid-19C+] to beat up; to assault; thus downer, a knock-down or knock-out blow.

2. [late 19C+] to defeat.

3. [1930s] to shoot dead.

In phrases

down blows in someone (v.) (also down a cuff/lash in someone)

[20C+] (W.I., Guyn.) to beat up severely.

down hand on someone (v.)

[20C+] (W.I.) to seize firmly.