'lexical' originates from Greek, specifically the word 'lexikos', where 'lexis' meant 'word'.

'lexical' passed into Late Latin/Medieval Latin as 'lexicalis' and then into English (Middle English) as 'lexical'; 'lexically' was formed by adding the adverbial suffix '-ly' (from Old English '-lic'/'-ly').

Initially it meant 'of or relating to words'; over time this core meaning has been retained, and 'lexically' has come to mean 'in terms of words or vocabulary.'