From Latin 'emissarius' (one sent out) — originally a scout or spy. Same root as 'mission,' 'missile,' and 'emit.'
A person sent on a special mission, usually as a diplomatic representative; an agent sent to represent another's interests.
From Latin ēmissārius (a person sent out, a scout, a spy, a secret agent), from ēmissus, past participle of ēmittere (to send out, to discharge, to release), composed of ex- (out) + mittere (to send, to let go). The Latin mittere derives from PIE *meyth₂- (to exchange, to go, to move), connected to English miss (to send a missile that fails its mark), dismiss, mission, missile, and permit. In classical Latin, ēmissārius carried a connotation of covert activity — it frequently named a spy or secret
In Roman hydraulic engineering, an 'emissarium' was a drainage outlet — a channel that 'sent out' water from a lake or reservoir. The famous Emissario del Fucino, built by Emperor Claudius in 52 CE, was a tunnel that drained Lake Fucino in central Italy. The word thus connected human agents sent out on missions and water sent out through channels — both are things