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1.
old school slang for let's get out of here!
I think we need to leave. (say it so everyone can hear) posse up!
by shiesty February 09, 2003
 
2.
Posse up -When a large number of people group together; get together to do a common thing; especially for retaliation.

Dey shot at my boy last night. We gotta posse up and git dem!
Posse- group of similar thinking people; from da same hood, school; apartment complex, family or job. Posse up- when uh gang, friends, or groups of people get together to do something as uh whole. For example; fight; especially, when it's in payback for something; Also rob, party, or steal as a group.
by Longfellarette November 27, 2012