word-forming element meaning "language," from Attic Greek glōtto-, from glōtta, variant of glōssa "tongue; language" (see gloss (n.2)).
before vowels, gluc-, word-forming element used since c. 1880s, a later form of glyco-, from Greek glykys "sweet," figuratively "delightful; dear; simple, silly," from *glku-, a dissimilation in Greek from PIE root *dlk-u- "sweet" (source also of Latin dulcis). De Vaan writes that "It is likely that we are dealing with a common borrowing from an unknown source." Now usually with reference to glucose.
before vowels gnath-, word-forming element meaning "jaw, mouth part, beak (of a bird)," from Greek gnathos "jaw," from PIE root *genu- (2) "jawbone, chin."
The extension of the sense to corresponding relationships of descent, "a generation younger than" (grandson, granddaughter) is from Elizabethan times. The inherited PIE root, *nepot- "grandchild" (see nephew) has shifted to "nephew; niece" in English and other languages (Spanish nieto, nieta). Old English used suna sunu ("son's son"), dohtor sunu ("son's daughter").