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]

The Collaborative International Dictionary of English v.0.48:

Sneaking \Sneak"ing\, a.
   Marked by cowardly concealment; deficient in openness and
   courage; underhand; mean; crouching. -- Sneak"ing*ly, adv.
   -- Sneak"ing*ness, n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sneaking
    adj 1: not openly expressed; "a sneaking suspicion" [syn:
           sneaking(a), unavowed]

Moby Thesaurus II by Grady Ward, 1.0:

85 Moby Thesaurus words for "sneaking":
   all fours, back-door, backstairs, clandestine, covert, cowering,
   crawl, crawling, creep, creeping, cringing, deep-rooted,
   deep-seated, devious, doggo, duplicitous, feline, furtive,
   guileful, gumshoeing, gut, hidden, hidden out, hidlings,
   hole-and-corner, hugger-mugger, in ambush, in hiding, in the wings,
   indirect, inherent, innate, intuitive, lingering, lurking, nagging,
   niggling, nightwalking, on tiptoe, padding, persistent, private,
   privy, prowling, pussyfoot, pussyfooted, pussyfooting, quailing,
   quiet, scrabble, scramble, secret, shifty, sidling, skulking,
   slinking, slinky, sly, snaking, sneaky, stealing, stealthy,
   suppressed, surreptitious, tippytoe, tiptoe, tiptoeing, undeclared,
   under cover, under-the-counter, under-the-table, undercover,
   underground, underhand, underhanded, undisclosed, undivulged,
   unexpressed, unobtrusive, unrevealed, unvoiced, waiting concealed,
   worming, worrisome, worrying