The term is "posse comitatus", straight Medieval Latin, and like most Medieval Latin a little screwy, with some explanation required, which I now attempt.
Posse is literally "to be able" (~"to have ability, power"), somewhat bent from its classical meaning. Here it's used as a collective noun for a group of men imbued with the power of the "county".
Comitatus originally meant "accompaniment, company, group", but in the Middle Ages was bent to mean the fiefdom of a count, or Latin comes, comitis (originally, "companion", in the Later Empire a kind of imperial general). We now call such a fief a "county" in the original English sense ("county", "earldom", "principality", "barony", and so on).
So a "posse comitatus" means "the power of the county" as reposed in the "posse" which assisted the sheriff ("shire riever") in administering the count's county.
The point of the argument being that the U.S. military is not imbued with posse comitatus under the Consitution, which reserves the enforcement of the laws of a State to the officers thereof, and to those of the municipalities (counties, cities) of which the State is made up.
And just to be annoying to liberals .... it was the practice, in the 19th century, for state and municipal officers to enforce federal law as well, a fact which bore critically on the task of keeping the size and expense of the federal government well short of outrageousness.
Good Post, relative to the Latin origin of Posse Commitas. I know what it means today but was not aware of how the meaning was derived from Latin.
As always, an informative and succinct post.