It is often used from the people called 'roadmen'.
It means to shoot up a group or gang .
In the song 'redrum reverse' from R6 67, it is used to show a threatening fallacy to express their anger.
When someone says or does something that everyone else considers stupid or embarassing, most of the time, someone will speak up and say "Can I get 'em?" meaning, "Can I get ..." Well, actually, no one really knows what it is we're "getting," but people find it very entertaining to say. And if they're very ghetto, they just might take it a step further and actually swipe their hand across the back of the persons' neck to symbolize them actually "getting" something. And there you have it.
*John runs into a pole accidentally.*
"HAHAHA! Can I get 'em?"
The act of throwing a plasma grenade at an unsuspecting person in Halo, resulting in them realizing it, freaking out, and eventually the joyous sight of them exploding with a dying "GRAWWWWWWWW"