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.).

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Definitions of sneak from WordNet
sneak (v.)
to go stealthily or furtively;
..stead of sneaking around spying on the neighbor's house
Synonyms: mouse / creep / pussyfoot
sneak (v.)
put, bring, or take in a secretive or furtive manner;
sneak a look
sneak a cigarette
sneak (v.)
make off with belongings of others;
Synonyms: pilfer / cabbage / purloin / pinch / abstract / snarf / swipe / hook / filch / nobble / lift
sneak (v.)
pass on stealthily;
Synonyms: slip
sneak (n.)
a person who is regarded as underhanded and furtive and contemptible;
sneak (n.)
someone who prowls or sneaks about; usually with unlawful intentions;
Synonyms: prowler / stalker
sneak (n.)
someone acting as an informer or decoy for the police;
Synonyms: fink / snitch / snitcher / stoolpigeon / stool pigeon / stoolie / sneaker / canary
sneak (adj.)
marked by quiet and caution and secrecy; taking pains to avoid being observed;
a sneak attack