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Greek lexis 'word, phrase'
c.1600, "a dictionary," from Middle French lexicon or directly from Modern Latin lexicon, from Greek lexikon (biblion) "word (book)," from neuter of lexikos "pertaining to words," from lexis "word," from legein "say" (see lecture (n.)).
Used originally of dictionaries of Greek, Syriac, Hebrew and Arabic, because these typically were in Latin and in Modern Latin lexicon, not dictionarius, was the preferred word. The modern sense of "vocabulary proper to some sphere of activity" (1640s) is a figurative extension.