What is the X in X-mas?
Old English smeortan "be painful," from Proto-Germanic *smarta- (cf. Middle Dutch smerten, Dutch smarten, Old High German smerzan, German schmerzen "to pain," originally "to bite"), from PIE *smerd- "pain," an extension of the root *mer- (2) "to rub; to harm" (cf. Greek smerdnos "terrible, dreadful," Sanskrit mardayati "grinds, rubs, crushes," Latin mordere "to bite"). Related: Smarted; smarting.
late Old English smeart "painful, severe, stinging; causing a sharp pain," related to smeortan (see smart (v.)). Meaning "executed with force and vigor" is from c.1300. Meaning "quick, active, clever" is attested from c.1300, from the notion of "cutting" wit, words, etc., or else "keen in bargaining." Meaning "trim in attire" first attested 1718, "ascending from the kitchen to the drawing-room c.1880" [Weekley]. For sense evolution, cf. sharp (adj.).
In reference to devices, the sense of "behaving as though guided by intelligence" (e.g. smart bomb) first attested 1972. Smarts "good sense, intelligence," is first recorded 1968. Smart cookie is from 1948.
"sharp pain," c.1200, from sharp (adj.). Cf. cognate Middle Dutch smerte, Dutch smart, Old High German smerzo, German Schmerz "pain."
The quality of something ''smarmy'': How to write pet stories, then, while skirting the swamps of smarm? (1937+)