Sneak

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late 16th century: probably dialect; perhaps related to obsolete snike ‘to creep’.

wiktionary

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Possibly from Middle English sniken(“to creep, crawl”), from Old English snīcan(“to creep, crawl”), from Proto-West Germanic *snīkan, from Proto-Germanic *snīkaną(“to creep, crawl”), which is related to the root of snake. Compare Danish snige(“to sneak”), Swedish snika(“to sneak, hanker after”), Icelandic sníkja(“to sneak, hanker after”).

etymonline

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sneak (v.)

1550s (implied in sneakish), perhaps from some dialectal survival of Middle English sniken "to creep, crawl" (c. 1200), related to Old English snican "to sneak along, creep, crawl," from Proto-Germanic *sneikanan, which is related to the root of snake (n.). Of feelings, suspicions, etc., from 1748. Transitive sense, "to partake of surreptitiously" is from 1883. Related: Sneaking. Sneak-thief is recorded by 1859; sneak-preview is from 1938.

sneak (n.)

"a sneaking person; mean, contemptible fellow," 1640s, from sneak (v.).