The Collaborative International Dictionary of English v.0.48:

Sneaky \Sneak"y\, n.
   Like a sneak; sneaking.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sneaky
    adj 1: marked by deception; "achieved success in business only
           by underhand methods" [syn: sneaky, underhand,
           underhanded]
    2: 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]

Moby Thesaurus II by Grady Ward, 1.0:

92 Moby Thesaurus words for "sneaky":
   Machiavellian, Machiavellic, acute, arch, artful, astute,
   back-door, backstairs, cagey, calculating, canny, chiseling,
   clandestine, clever, collusive, covert, covinous, cowering, crafty,
   cringing, cunning, cute, deceitful, deep, deep-laid, designing,
   devious, diplomatic, dishonest, disingenuous, duplicitous, false,
   falsehearted, feline, finagling, foxy, fraudulent, furtive,
   guileful, hidlings, hole-and-corner, hugger-mugger, indirect,
   ingenious, insidious, inventive, knowing, pawky, politic, privy,
   quailing, quiet, ready, resourceful, scheming, serpentine, sharp,
   shifty, shrewd, skulking, slick, slinking, slinky, slippery, sly,
   smooth, snaky, sneaking, sophistical, stealthy, strategic, subtile,
   subtle, supple, surreptitious, tactical, treacherous, trickish,
   tricksy, tricky, two-faced, under-the-counter, under-the-table,
   undercover, underground, underhand, underhanded, unobtrusive,
   unscrupulous, vulpine, wary, wily