What is the X in X-mas?
early 15c., variant of Middle English slidder "to slip, slide," from Old English slidrian "to slip, slide on a loose slope," a frequentative form of slidan "to slide" (see slide (v.)). For spelling change, cf. gather. Meaning "to walk in a sliding manner" is attested from 1848. In reference to reptile motion, attested from 1839. Related: Slithered; slithering.
"slithering movement," 1861, from slither (v.).