Yours, Etc.: Origins and Uses of 8 Sign-Offs
c.1300, "state of being acquainted;" late 14c., "person with whom one is acquainted;" also "personal knowledge;" from Old French acointance "acquaintance, friendship, familiarity," noun of action from acointer (see acquaint). Acquaintant (17c.), would have been better in the "person known" sense but is now obsolete.