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"luster," 1530s, from Scandinavian (cf. Icelandic glossi "flame," related to glossa "to flame"), or obsolete Dutch gloos "a glowing," from Middle High German glos; probably ultimately from the same source as Old English glowan (see glow (v.)).
"word inserted as an explanation," 1540s (earlier gloze, c.1300), from Latin glossa "obsolete or foreign word," one that requires explanation; hence also "explanation, note," from Greek glossa (Ionic), glotta (Attic) "obscure word, language," also "mouthpiece," literally "tongue," from PIE *glogh- "thorn, point, that which is projected" (cf. Old Church Slavonic glogu "thorn"). Figurative use from 1540s. Both glossology (1716) and glottology (1841) have been used in the sense "science of language."
1570s as "insert a word as an explanation," from gloss (n.2). From 1650s as "to add luster," from gloss (n.1). Figurative sense of "smooth over, hide" is from 1729, mostly from gloss (n.1) but showing influence of gloss (n.2) in the extended verbal sense of "explain away" (1630s), from idea of a note inserted in the margin of a text to explain a difficult word. Related: Glossed; glossing.