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 first recorded 1859; sneak-preview is from 1938.
sneak (n.)
"a sneaking person; mean, contemptible fellow," 1640s, from sneak (v.).