Slither

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Middle English: alteration of the dialect verb slidder, frequentative from the base of slide.

wiktionary

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From Middle English slitheren, alteration of slideren(“to slither, creep”), from Old English slidrian(“to slip, slide, slither”), from Proto-West Germanic *slidrōn(“to slide, slither”), from Proto-Indo-European *sleydʰ-(“to slip”), equivalent to slide +‎ -er(frequentative suffix). Cognate with Dutch slidderen(“to slip, wriggle, slither”), German schlittern(“to slither, skid”). More at slide.

etymonline

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slither (v.)

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, compare gather. Meaning "to walk in a sliding manner" is attested from 1848. In reference to reptile motion, attested from 1839. Related: Slithered; slithering.

slither (n.)

"slithering movement," 1861, from slither (v.).