"word inserted as an explanation, translation, or definition," c. 1300, glose (modern form from 1540s; earlier also gloze), from Late Latin glossa "obsolete or foreign word," one that requires explanation; later extended to the explanation itself, from Greek glossa (Ionic), glotta (Attic) "language, a tongue; word of mouth, hearsay," also "obscure or foreign word, language," also "mouthpiece," literally "the tongue" (as the organ of speech), from PIE *glogh- "thorn, point, that which is projected" (source also of Old Church Slavonic glogu "thorn," Greek glokhis "barb of an arrow").
Glosses were common in the Middle Ages, usually rendering Hebrew, Greek, or Latin words into vernacular Germanic, Celtic, or Romanic. Originally written between the lines, later in the margins. By early 14c. in a bad sense, "deceitful explanation, commentary that disguises or shifts meaning." This sense probably has been colored by gloss (n.1). Both glossology (1716) and glottology (1841) have been used in the sense "science of language."