ambasadoro

Another -or- fossil, and it is worth naming the class. Esperanto makes agent nouns with -ist- for a profession and -ul- for a person characterised by something. It has no -or- suffix at all. So ambasadoro is not ambasad- plus an agent ending in any Esperanto sense - there is no root ambasad- to strip it back to. The word came in already assembled, Latin morphology and all, exactly like motoro, redaktoro, doktoro, profesoro and direktoro. When a root ends in -oro and refuses to divide, this is usually why.

The proof is in the embassy. If -or- were a suffix here, the building would be ambasadejo. It is not: it is ambasadorejo, the place of the ambasadoro, with -ej- attached to the whole undivided root. Esperanto's own morphology tells you where the seam is, if you watch what the language does rather than what the word looks like.

The job, and the borrowed sense. An ambasadoro represents one state to another, outranking a konsulo and a mere sendito or envoy; ambasadorino is used where the sex is worth marking, and the modern habit is simply to leave it off. Esperanto also borrows the figurative sense English uses: a kultura ambasadoro, or - and the movement says this about itself often enough - every speaker as an ambasadoro de la lingvo.