Prose
The word "prose" first appears in English in the 14th century. It is derived from the Old French "prose", which in turn originates in the Latin expression "prosa oratio" (literally, straightforward or direct speech).
Posse comitatus
The term derives from the Latin "posse comitātūs," "power" or "force of the county", in English use from the late 16th century, shortened to posse from the mid 17th century.
While the original meaning refers to a group of citizens assembled by the authorities to deal with an emergency (such as suppressing a riot or pursuing felons), the term is also used for any force or band, especially with hostile intent, often also figuratively or humorously. In 19th-century usage, "posse comitatus" also acquires the generalized or figurative meaning. In classical Latin, posse is a contraction of “potesse,” an irregular, subjunctive Latin verb meaning “to be able.”    Thus when paired with comitātus, “to accompany,”  posse comitatus literally means: “to be able to accompany.”   In its earliest days the posse comitatus was subordinate to king, country and local authority.