What is the X in X-mas?
late 14c., originally in reference to the sun's apparent motion in the sky, from Old French arc "bow, arch, vault" (12c.), from Latin arcus "a bow, arch," from PIE root *arku- "bowed, curved" (cf. Gothic arhvazna "arrow," Old English earh, Old Norse ör; also, via notion of "supple, flexible," Greek arkeuthos, Latvian ercis "juniper," Russian rakita, Czech rokyta, Serbo-Croatian rakita "brittle willow"). Electrical sense is from 1821.
1893, in the electrical sense, from arc (n.). Meaning "to move in an arc" attested by 1954. Related: Arced; arcing.
A curved line or segment of a circle.