The Collaborative International Dictionary of English v.0.48:

Sly \Sly\, a. [Compar. Slieror Slyer; superl. Sliest or
   Slyest.] [OE. sli, slegh, sleih, Icel. sl?gr, for sl?gr;
   akin to Sw. slug, Dan. slu, LG. slou, G. schlau; probably to
   E. slay, v.t.; cf. G. verschlagen sly. See Slay, v. t., and
   cf. Sleight.]
   1. Dexterous in performing an action, so as to escape notice;
      nimble; skillful; cautious; shrewd; knowing; -- in a good
      sense.
      [1913 Webster]

            Be ye sly as serpents, and simple as doves. --Wyclif
                                                  (Matt. x. 16).
      [1913 Webster]

            Whom graver age
            And long experience hath made wise and sly.
                                                  --Fairfax.
      [1913 Webster]

   2. Artfully cunning; secretly mischievous; wily.
      [1913 Webster]

            For my sly wiles and subtle craftiness,
            The litle of the kingdom I possess.   --Spenser.
      [1913 Webster]

   3. Done with, and marked by, artful and dexterous secrecy;
      subtle; as, a sly trick.
      [1913 Webster]

            Envy works in a sly and imperceptible manner. --I.
                                                  Watts.
      [1913 Webster]

   4. Light or delicate; slight; thin. [Obs.]
      [1913 Webster]

   By the sly, or On the sly, in a sly or secret manner.
      [Colloq.] "Gazed on Hetty's charms by the sly." --G.
      Eliot.

   Sly goose (Zool.), the common sheldrake; -- so named from
      its craftiness.
      [1913 Webster]

   Syn: Cunning; crafty; subtile; wily. See Cunning.
        [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Sly \Sly\, adv.
   Slyly. [Obs. or Poetic] --Spenser.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sly
    adj 1: marked by skill in deception; "cunning men often pass for
           wise"; "deep political machinations"; "a foxy scheme"; "a
           slick evasive answer"; "sly as a fox"; "tricky Dick"; "a
           wily old attorney" [syn: crafty, cunning, dodgy,
           foxy, guileful, knavish, slick, sly, tricksy,
           tricky, wily]

Moby Thesaurus II by Grady Ward, 1.0:

98 Moby Thesaurus words for "sly":
   Byzantine, Machiavellian, Machiavellic, acute, adroit, arch,
   artful, astute, back-door, backstairs, cagey, calculating, canny,
   clandestine, clever, covert, crafty, creep, crooked, cunning, cute,
   deceitful, deep, deep-laid, designing, devious, dexterous,
   diplomatic, dishonest, disingenuous, downy, feline, foxy, furtive,
   guileful, gumshoe, hidlings, hole-and-corner, hugger-mugger,
   ingenious, insidious, inventive, knowing, lurk, masterful, pawky,
   politic, predatory, privy, quiet, ready, resourceful, scheming,
   serpentine, shady, sharp, shifty, shrewd, skulk, skulking, slick,
   slide, slim, slink, slinking, slinky, slippery, smart, smooth,
   snake, snaky, sneaking, sneaky, snook, sophistical, steal,
   stealthy, strategic, subtile, subtle, supple, surreptitious,
   tactical, trickish, tricksy, tricky, under-the-counter,
   under-the-table, undercover, underground, underhand, underhanded,
   unfrank, unobtrusive, unscrupulous, vulpine, wary, wily