Webster's Revised Unabridged Dictionary · 1913

Posse comitatus

Posse comitatus

[Latin posse to be able, to have power + Late Latin comitatus a county, from comes, comitis, a count. See County, and Power.]

1.
(Law) The power of the county, or the citizens who may be summoned by the sheriff to assist the authorities in suppressing a riot, or executing any legal precept which is forcibly opposed. — Blackstone
2.
A collection of people; a throng; a rabble. [Colloquial]
As if the passion that rules were the sheriff of the place, and came off with all the posse. — Locke

The word comitatus is often omitted, and posse alone used. “A whole posse of enthusiasts.”