What is the X in X-mas?
Old English wearm, from Proto-Germanic *warmaz (cf. Old Saxon, Old Frisian, Middle Dutch, Old High German, German warm, Old Norse varmr, Gothic warmjan "to warm"), from PIE *gwher- (cf. Sanskrit gharmah "heat;" Old Persian Garmapada-, name of the fourth month, corresponding to June/July, from garma- "heat;" Armenian jerm "warm;" Greek thermos "warm;" Latin formus "warm," fornax "oven;" Old Irish fogeir "heated;" Hittite war- "to burn"). The root also may be connected to that of Old Church Slavonic goriti "to burn," varu "heat," variti "to cook, boil;" and Lithuanian verdu "to seethe."
The distinction, based on degree of heat, between "warm" and "hot" is general in Balto-Slavic and Germanic, but in other languages one word often covers both (cf. Latin calidus, Greek thermos, French chaud, Spanish caliente). In reference to feelings, etc., attested from late 15c. Sense in guessing games first recorded 1860, from earlier hunting use in reference to scent or trail (1713). Warm-blooded in reference to mammals is recorded from 1793. Warm-hearted first recorded c.1500.
Old English wyrman "make warm" and wearmian "become warm;" from the root of warm (adj.). Phrase warm the bench is sports jargon first recorded 1907. Warm up (v.) "exercise before an activity" is attested from 1868. In reference to appliances, motors, etc., attested from 1947. Noun phrase warm-up "act or practice of warming up" is recorded from 1915. Related: Warmed; warming.
SCOTCH WARMING PAN. A wench. [Grose, "Dictionary of the Vulgar Tongue," 1788]