The Collaborative International Dictionary of English v.0.48:

Sneak \Sneak\, v. t.
   To hide, esp. in a mean or cowardly manner. [Obs.] "[Slander]
   sneaks its head." --Wake.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Sneak \Sneak\, n.
   1. A mean, sneaking fellow.
      [1913 Webster]

            A set of simpletons and superstitious sneaks.
                                                  --Glanvill.
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   2. (Cricket) A ball bowled so as to roll along the ground; --
      called also grub. [Cant] --R. A. Proctor.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Sneak \Sneak\ (sn[=e]k), v. i. [imp. & p. p. Sneaked
   (sn[=e]kt); p. pr. & vb. n. Sneaking.] [OE. sniken, AS.
   sn[imac]can to creep; akin to Dan. snige sig; cf. Icel.
   sn[imac]kja to hanker after.]
   1. To creep or steal (away or about) privately; to come or go
      meanly, as a person afraid or ashamed to be seen; as, to
      sneak away from company.
      [1913 Webster]

            You skulked behind the fence, and sneaked away.
                                                  --Dryden.
      [1913 Webster]

   2. To act in a stealthy and cowardly manner; to behave with
      meanness and servility; to crouch.
      [1913 Webster]

WordNet (r) 3.0 (2006):

sneak
    adj 1: marked by quiet and caution and secrecy; taking pains to
           avoid being observed; "a furtive manner"; "a sneak
           attack"; "stealthy footsteps"; "a surreptitious glance at
           his watch" [syn: furtive, sneak(a), sneaky,
           stealthy, surreptitious]
    n 1: a person who is regarded as underhanded and furtive and
         contemptible
    2: someone who prowls or sneaks about; usually with unlawful
       intentions [syn: prowler, sneak, stalker]
    3: someone acting as an informer or decoy for the police [syn:
       fink, snitch, snitcher, stoolpigeon, stool pigeon,
       stoolie, sneak, sneaker, canary]
    v 1: to go stealthily or furtively; "..stead of sneaking around
         spying on the neighbor's house" [syn: sneak, mouse,
         creep, pussyfoot]
    2: put, bring, or take in a secretive or furtive manner; "sneak
       a look"; "sneak a cigarette"
    3: make off with belongings of others [syn: pilfer, cabbage,
       purloin, pinch, abstract, snarf, swipe, hook,
       sneak, filch, nobble, lift]
    4: pass on stealthily; "He slipped me the key when nobody was
       looking" [syn: slip, sneak]

Moby Thesaurus II by Grady Ward, 1.0:

84 Moby Thesaurus words for "sneak":
   arrant coward, blackguard, caitiff, clandestine, couch, covert,
   cower, craven, crawl, creep, cringe, crouch, cur, dastard, devil,
   fink, furtive, glide, go on tiptoe, grass, grovel, gumshoe, heel,
   hole-and-corner, hush-hush, inch, inch along, informer, knave,
   lay wait, lie in wait, louse, lurk, nark, nightwalk, pad, poltroon,
   precious rascal, prowl, pussyfoot, quail, rapscallion, rascal,
   recreant, reptile, rogue, run, scalawag, scamp, scoundrel,
   scrabble, scramble, shadow, shirk, shithead, shyster, sidle, skulk,
   skunk, slide, slink, slip, slither, sly, smuggle, snake, sneaker,
   snitch, snook, spalpeen, stalk, steal, steal along, stealthy,
   stoolie, surreptitious, tippytoe, tiptoe, toad, undercover,
   villain, weasel, worm, worm along