Sneak  —v. 1 (foll. By in, out, past, away, etc.) Go or convey furtively. 2 slang steal unobserved. 3 slang tell tales; turn informer. 4 (as sneaking adj.) A furtive (sneaking affection). B persistent and puzzling (sneaking feeling). —n. 1 mean-spirited underhand person. 2 slang tell-tale; informer. —adj. Acting or done without warning; secret.  sneaky adj. (-ier, -iest). [origin uncertain]