proprietor 1630s, "owner, by royal grant, of an American colony," probably from proprietary (n.) in sense "property owner" (late 15c., see proprietary). In general sense of "one who holds something as property" it is attested from 1640s.
proprietary mid-15c., "possessing worldly goods in excess of a cleric's needs," from M.L. proprietarius "owner of property," noun use of L.L. adj. proprietarius "of a property holder," from L. proprietas "owner" (see property). Meaning "held in private ownership" is first attested 1580s.
praetor magistrate in ancient Rome (next in rank to consuls), early 15c., from L. praetor "one who goes before, a consul as leader of an army," from prae "before" (see pre-) + root of ire "to go" (see ion).