mid-15c., of weapons, "strike obliquely without giving full impact," a nasalized form of glacen "to graze, strike a glancing blow" (c. 1300), from Old French glacier "to slip, make slippery" (compare Old French glaciere "part of a knight's armor meant to deflect blows"), from glace "ice" (see glacial). Sense of "look quickly" (first recorded 1580s) probably was by influence of Middle English glenten "look askance" (see glint (v.)), which also could account for the -n-. Related: Glanced; glancing.
c. 1500, "a sudden movement producing a flash," from glance (v.). Meaning "brief or hurried look" is from 1590s.