means A group of people gathered to back someone up — historically a band deputized to help enforce the law, now any loyal crew that rolls with you.

from From the Latin phrase posse comitatus, 'the force of the county' — posse being the verb 'to be able, to have power.' English law shortened it to just posse, the body of citizens a sheriff could legally summon to chase down a wrongdoer. The American West turned that legal mechanism into pure cinema, and the word eventually drifted from courthouse to clubhouse, picked up by hip-hop and slang to mean your tight-knit gang of allies.