What is the X in X-mas?
mid-15c., of weapons, from glacen "to graze, strike a glancing blow" (c.1300), from Old French glacier "to slip, make slippery," from glace "ice" (see glacial). Sense of "look quickly" (first recorded 1580s) probably was influenced in form and meaning by Middle English glenten "look askance" (see glint). Related: Glanced; glancing.
c.1500, "sudden movement producing a flash," from glance (v.). Meaning "brief or hurried look" is from 1590s.