Is it farther or further?
late 13c., "shine brightly," from or related to Middle Dutch, Middle Low German glaren "to gleam," related by rhoticization to glas (see glass). Sense of "stare fiercely" is from late 14c. The noun is c.1400 in sense "bright light;" 1660s in sense of "fierce look." Old English glær (n.) meant "amber." Related: Glared; glaring.