To serve as an ambassador or to conduct diplomatic relations and negotiations.
Derived from 'ambassador' plus the verb-forming suffix '-ate.' The root 'ambassador' comes from Old French 'ambassadeur,' ultimately traceable to 'ambassage' (mission or message) described above.
This is an extremely rare verb that appears mostly in historical and diplomatic texts—it means to basically be an ambassador or do ambassador work, but it's so uncommon that modern diplomats never use it, preferring to just say 'represent' or 'negotiate' instead.
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