What does the word "gloss" mean in Greek?

The English word gloss comes from ancient Greek glossa, written as γλῶσσα, which has two primary meanings that explain its modern senses. The most literal meaning of glossa in Greek is tongue, the physical organ in the mouth. By extension, it came to mean language, speech, or tongue in the sense of a particular language or dialect, as in someone speaking a different tongue. This is where we get medical terms like glossitis meaning inflammation of the tongue and glossopharyngeal referring to tongue and throat. The second major meaning of glossa in Greek developed from the first. It came to mean a difficult, obscure, archaic, foreign, or dialectal word that needs explanation, because such a word would be unfamiliar to the average speaker and would require a tongue or language explanation. This led to the scholarly practice of writing explanations for such words, which were called glossai. From this sense we get glossary, glossator, and the verb to gloss meaning to explain. So in Greek, gloss fundamentally means tongue and by extension language and a word needing explanation, which is why we have both medical tongue terms and literary explanation terms from the same root.