Within the context of cussing out someone, out-talking/smarting them; all in the sense to make them look stupid or wrong.
Literally "restrains" them from having
anything else to say.
Origin:
(1) Cappin like a
bottle cap, shuttin you up.
(2) Breakin yo legs, handi"cappin" yo ass, you is now defenseless, can't do any other shit.