posse mobilitatis, n. — Green’s Dictionary of Slang

posse mobilitatis n.

[play on Lat. posse comitatus, force of the county, a band of citizens summoned by the sheriff to deal with outbreaks of rioting and similar disorder]

the mob.

c.1698		B.E. Dict. Canting Crew n.p.: Posse Mobilitatis the whole Rabble in a Body.
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue.
1811		Lex. Balatronicum.