1640s (in Anglo-Latin from early 14c.), shortening of posse comitatus "the force of the county" (1620s, in Anglo-Latin from late 13c.), from Medieval Latin posse "body of men; power," from Latin posse "have power, be able" (see potent) + comitatus "of the county," genitive of Late Latin word for "court palace" (see comitatus). General sense of "an armed force" is from 1640s; the modern slang meaning "small gang" probably is from Western movies.
Posse comitatus, the power of the county; in law, the body of men which the sheriff is empowered to call into service to aid and support him in the execution of the law, as in case of rescue, riot, forcible entry and occupation, etc. It includes all male persons above the age of fifteen. In Great Britain peers and clergymen are excluded by statute. The word comitatus is often omitted, and posse alone is used in the same sense. [Century Dictionary]