Webster's Revised Unabridged Dictionary · 1913

Sly

Sly , adjective

[Old English sli, slegh, sleih, Icelandic sl{not transcribed}gr, for sl{not transcribed}gr; akin to Swedish slug, Danish slu, LG. slou, German schlau; probably to English slay, transitive verb; compare German verschlagen sly. See Slay, transitive verb, and compare 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. [Obsolete]
Collocations (2)
By the sly or On the sly , in a sly or secret manner. [Colloquial] Gazed on Hetty's charms by the sly. — G. Eliot
Sly goose (Zoology) , the common sheldrake; -- so named from its craftiness.

Sly , adverb

Slyly. [Obsolete or Poetic] — Spenser