c. 1300, purpus, "intention, aim, goal; object to be kept in view; proper function for which something exists," from Anglo-French purpos, Old French porpos "an aim, intention" (12c.), from porposer "to put forth," from por- "forth" (from a variant of Latin pro- "forth;" see pur-) + Old French poser "to put, place" (see pose (v.1)).
Etymologically it is equivalent to Latin propositium "a thing proposed or intended," but evidently formed in French from the same elements. From mid-14c. as "theme of a discourse, subject matter of a narrative (as opposed to digressions), hence to the purpose "appropriate" (late 14c.). On purpose "by design, intentionally" is attested from 1580s; earlier of purpose (early 15c.).
late 14c., purposen, "to intend (to do or be something); put forth for consideration, propose," from Anglo-French purposer "to design," Old French purposer, porposer "to intend, propose," variant of proposer "propose, advance, suggest" (see propose).
Generally with an infinitive. Intransitive sense of "to have intention or design" is by mid-15c. According to Century Dictionary, "The verb should prop. be accented on the last syllable (as in propose, compose, etc.), but it has conformed to the noun," which is wholly from Latin while the verb is partly of different origin (see pose (n.2)).