ambassador: [14] Appropriately enough, ambassador is a highly cosmopolitan word. It was borrowed back and forth among several European languages before arriving in English. Its ultimate source appears to be the Indo- European root *ag- ‘drive, lead’, whose other numerous offspring include English act and agent. With the addition of the prefix *amb- ‘around’ (as in ambidextrous), this produced in the Celtic languages of Gaul the noun ambactos, which was borrowed by Latin as ambactus ‘vassal’.
The Latin word then found its way into the Germanic languages – Old English had ambeht ‘servant, messenger’, Old High German ambaht (from which modern German gets amt ‘official position’) – from which it was later borrowed back into medieval Latin as ambactia. This seems to have formed the basis of a verb, *ambactiāre ‘go on a mission’ (from which English ultimately gets embassy), from which in turn was derived the noun *ambactiātor.
This became ambasciator in Old Italian, from which Old French borrowed it as ambassadeur. The word had a be wildering array of spellings in Middle English (such as ambaxadour and inbassetour) before finally settling down as ambassador in the 16th century. => embassy
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.