Source : Webster's Revised Unabridged Dictionary (1913)

Sly \Sly\, a. [Compar. {Slier}or {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.

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

            Whom graver age And long experience hath made wise
            and sly.                              --Fairfax.

   2. Artfully cunning; secretly mischievous; wily.

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

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

            Envy works in a sly and imperceptible manner. --I.
                                                  Watts.

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

   {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} (Zo["o]l.), the common sheldrake; -- so named
      from its craftiness.

   Syn: Cunning; crafty; subtile; wily. See {Cunning}.

Source : WordNet®

on the sly
     adv : in a furtive manner; "the soldiers were furtively crawling
           through the night" [syn: {furtively}]