mid-14c., "to shine;" c.1500, "to stare with wide eyes," perhaps from a Scandinavian source (cf. Norwegian dialectal glora "to glow"), from Proto-Germanic base *glo- (see glow (v.)), root of Old English glowan "to glow," which influenced the spelling. Or perhaps related to Middle Dutch gluren "to leer." Meaning "to look angrily, scowl" is first recorded 1775. Related: Glowered; glowering. As a noun, 1715, from the verb.