What is the X in X-mas?
Old English swætan "perspire, work hard," from the source of sweat (n.). Meaning "to be worried, vexed" is recorded from c.1400. Related: Sweated; sweating. Colloquial no sweat "no problem" attested from 1963.
Old English swat "sweat," which became Middle English swote, but altered under the influence of the verb, from Proto-Germanic *swaita (cf. Old Saxon, Old Frisian swet, Old Norse sveiti, Danish sved "sweat," Swedish svett, Middle Dutch sweet, Dutch zweet, Old High German sweiz, German Schweiß), from PIE *sweid-/*swoid- (cf. Sanskrit svedah "sweat," Avestan xvaeda- "sweat," Greek hidros "sweat, perspiration," Latin sudor, Lettish swiedri, Welsh chwys "sweat"). Sweat equity is from 1968.
v. sweat·ed or sweat, sweat·ing, sweats
To excrete perspiration through the pores in the skin; perspire. n.
The colorless saline moisture excreted by the sweat glands; perspiration.
The process of sweating.
To affirm with absolute confidence and considerable vehemence: Don called all those short-term signals for Joe. I'd swear to that on a stack of Bibles (1866+)