What is "by the sly"

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.

   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.

adv. (context dated English) on the sly