What is the X in X-mas?
c.1300, "absence, want; shortage, deficiency," perhaps from an unrecorded Old English *lac, or else borrowed from Middle Dutch lak "deficiency, fault;" in either case from Proto-Germanic *laka- (cf. Old Frisian lek "disadvantage, damage," Old Norse lakr "lacking"), from PIE *leg- "to dribble, trickle." Middle English also had lackless "without blame or fault."
late 12c., perhaps from Middle Dutch laken "to be wanting," from lak (n.) "deficiency, fault," or an unrecorded native cognate word (see lack (n.)). Related: Lacked; lacking.
sticky, resinous secretion of the tiny lac insect, Laccifer lacca, which is a species of scale insect. This insect deposits lac on the twigs and young branches of several varieties of soapberry and acacia trees and particularly on the sacred fig, Ficus religiosa, in India, Thailand, Myanmar (Burma), and elsewhere in Southeast Asia. The lac is harvested predominantly for the production of shellac (q.v.) and lac dye, a red dye widely used in India and other Asian countries. Forms of lac, including shellac, are the only commercial resins of animal origin