Yours, Etc.: Origins and Uses of 8 Sign-Offs
mid-15c., "possessing worldly goods in excess of a cleric's needs," from Medieval Latin proprietarius "owner of property," noun use of Late Latin adjective proprietarius "of a property holder," from Latin proprietas "owner" (see property). Meaning "held in private ownership" is first attested 1580s. The word was used earlier in English as a noun meaning "proprietor," also "worldly person" (c.1400), from a noun use in French and Medieval Latin.
proprietary pro·pri·e·tar·y (prə-prī'ĭ-těr'ē)
Exclusively owned, as of a hospital.
Owned by an individual or corporation under a trademark or patent, as of a drug.