Found 2 items, similar to on the sly.
English → English (WordNet) Definition: on the sly on the sly adv : in a furtive manner; “the soldiers were furtively crawling through the night” [syn: furtively]
English → English (gcide) Definition: On 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. [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 (Zo["o]l.), the common sheldrake; -- so named from its craftiness. [1913 Webster] Syn: Cunning; crafty; subtile; wily. See Cunning. [1913 Webster]