late 14c., also embassador, "diplomatic emissary of a ruler in the court of another," from Old French embassator, ambassateor, which comes via Provençal or Old Spanish from Latin ambactus "a servant, vassal," from Celtic amb(i)actos "a messenger, servant" (from PIE root *ambhi- "around" + *ag- "to drive, draw out or forth, move").
Compare embassy. Forms in am- and em- were used indiscriminately in English 17c.-18c. Until 1893 the United States sent and received none, having only ministers (often called ambassadors), who represented the state, not the sovereign.