lexicon

English lexicon comes through Middle French or New Latin lexicon from Byzantine Greek lexikón, 'a lexicon, a dictionary.' That word was a shortened form of Ancient Greek lexikòn biblíon, literally 'a book of words,' from lexikós, 'of words,' from léxis, 'a saying, speech, word,' from légō, 'to speak,' ultimately from Proto-Indo-European *leǵ-, 'to gather, collect.'

The Ancient Greek phrase 'a book of words' was clipped down to just lexikón once everyone understood what kind of book was meant. In English, it's attested since 1583 meaning 'a dictionary of a classical language.'

It shares its root with léxis, 'a saying, speech, word,' and ultimately with légō, 'to speak' — the same deep root, Proto-Indo-European *leǵ-, meaning 'to gather, collect,' ties the idea of speaking to the idea of gathering things together.