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.
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            Be ye sly as serpents, and simple as doves. --Wyclif
                                                  (Matt. x. 16).
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            Whom graver age
            And long experience hath made wise and sly.
                                                  --Fairfax.
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   2. Artfully cunning; secretly mischievous; wily.
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            For my sly wiles and subtle craftiness,
            The litle of the kingdom I possess.   --Spenser.
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   3. Done with, and marked by, artful and dexterous secrecy;
      subtle; as, a sly trick.
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            Envy works in a sly and imperceptible manner. --I.
                                                  Watts.
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   4. Light or delicate; slight; thin. [Obs.]
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   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.
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   Syn: Cunning; crafty; subtile; wily. See Cunning.
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WordNet (r) 3.0 (2006):

on the sly
    adv 1: in a furtive manner; "the soldiers were furtively
           crawling through the night" [syn: furtively, on the
           sly]