"Form a posse" is a typical line from many 'B' Westerns. But where was the word "posse" imported from?
The term "posse" comes from Latin. The original phrase posse comitātūs meant "power or force of the county". The term was adopted by English speakers at the end of the 16th century and was shortened to posse in the middle of the 17th century.
In fact, the term is considered obsolete in Modern English, but it's still used in the United States to describe any law enforcement. While the original meaning refers to a group of citizens assembled by the authorities to deal with an emergency, the phrase is also often used to describe any group or force with hostile intent. Sometimes it occurs as a metaphorical term.