"glistening smoothness, luster," 1530s, probably from Scandinavian (compare Icelandic glossi "a spark, a flame," related to glossa "to flame"), or obsolete Dutch gloos "a glowing," from Middle High German glos; probably ultimately from the same source as English glow (v.). Superficial lustrous smoothness due to the nature of the material (unlike polish, which is artificial).
"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 glōssa (Ionic), glōtta (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."
c. 1300, glosen "use fair words; speak smoothly, cajole, flatter;" late 14c. as "comment on (a text), insert a word as an explanation, interpret," from Medieval Latin glossare and Old French gloser, from Late Latin glossa (see gloss (n.2)). Modern spelling from 16c.; formerly also gloze.
The other verb, meaning "to add luster, make smooth and shining," is from 1650s, from gloss (n.1). Figurative sense of "smooth over, hide" is from 1729, mostly from the first verb, in its extended sense of "explain away, veil or shift the meaning of," but showing influence of the second. Related: Glossed; glossing.