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1620s, from French émissaire (17c.) or directly from Latin emissarius, literally "that is sent out," from emissus, past participle of emittere "send forth" (see emit).
emissary em·is·sar·y (ěm'ĭ-sěr'ē)n. Any of various venous channels through the skull that connect the venous sinuses of the dura mater with veins external to the skull.