From Middle English prose, from Old French prose, from Latin prōsa (“straightforward”) from the term prōsa ōrātiō (“a straightforward speech – i.e. without the ornaments of verse”).
further etymology and related terms
The term prōsa (“straightforward”), a colloquial form of prorsa (“straight forwards”), the feminine form prorsus (“straight forwards”), from Old Latin prōvorsus (“moving straight ahead”), from pro- (“forward”) + vorsus (“turned”), form of vertō (“I turn”). Compare verse.