late 13c., "to shine brightly," from or related to Middle Dutch, Middle Low German glaren "to gleam," from Proto-Germanic *glaz-, from PIE root *ghel- (2) "to shine." Sense of "stare fiercely" is from late 14c. Related: Glared; glaring.
c. 1400, "bright light, dazzling glitter," from glare (v.); especially in reference to light reflected off some surface (17c.). From 1660s in sense of "fierce look." Old English glær (n.) meant "amber."