ambassador (n.)
late 14c., also embassador, from Middle French ambassadeur, from Old French embassator, via Provençal or Old Spanish from Latin ambactus "a servant, vassal," from Celtic amb(i)actos "a messenger, servant," from PIE *ambhi- "about" (see ambi-) + *ag- "drive, lead" (see act (v.)). Compare embassy. Forms in am- and em- were used indiscriminately 17c.-18c.