The Collaborative International Dictionary of English v.0.48:

Furtive \Fur"tive\, a. [L. furtivus, fr. furtum theft, fr. fur
   thief, akin to ferre to bear: cf. F. furtif. See Fertile.]
   Stolen; obtained or characterized by stealth; sly; secret;
   stealthy; as, a furtive look. --Prior.
   [1913 Webster]

         A hasty and furtive ceremony.            --Hallam.
   [1913 Webster]

WordNet (r) 3.0 (2006):

furtive
    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]
    2: secret and sly or sordid; "backstairs gossip"; "his low
       backstairs cunning"- A.L.Guerard; "backstairs intimacies";
       "furtive behavior" [syn: backstair, backstairs,
       furtive]

Moby Thesaurus II by Grady Ward, 1.0:

70 Moby Thesaurus words for "furtive":
   artful, back-door, backstairs, calculating, chiseling, clandestine,
   collusive, conspiratorial, covert, covinous, crafty, cunning,
   deceitful, doggo, false, falsehearted, feline, finagling, foxy,
   fraudulent, guileful, hidden, hidden out, hidlings,
   hole-and-corner, hugger-mugger, in ambush, in hiding, in the wings,
   indirect, insidious, lurking, on tiptoe, private, privy, prowling,
   pussyfoot, pussyfooted, quiet, scheming, secret, secretive, sharp,
   shifty, skulking, slinking, slinky, slippery, sly, sneaking,
   sneaky, stealing, stealthy, surreptitious, treacherous, trickish,
   tricky, two-faced, under cover, under the table, under-the-counter,
   under-the-table, undercover, underground, underhand, underhanded,
   unobtrusive, untrustworthy, waiting concealed, wily