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The act of acquiring firearms to go to war with a rival Gang, Crew, Set etc.
Used often in the series "The Wire"

Drug Dealer: Look man, all I'm saying if I'm gonna take you're package, split me and be fair. I got to pay all my people the same, and here you're upping the price on me.

Marlo Stanfield: Pay you're people less.

Drug Dealer: They my people, though.

Marlo Stanfield: Then short yourself... Split is 60/40. If you want better? You need to Tool Up and wait for Chris, Snoop, and the rest of my people, come pay a call on you're people.

Snoop: We will be brief with all you motherfuckers, I think you know.

Drug Dealer: A'ight. (Drug dealers walks away to go tool up).
by M I N N September 21, 2010
In any physical work environment where workers wear tool belts and occasionally remove them, this phrase is a call to arms (or to re-arm).
OK boys, break's over. Tool up!
by Chris Zizzo August 14, 2006
grab your pistol; get your gun.
yo son get the heater, dem boyz comin
by jgilla August 24, 2005