Artfully cunning; secretly mischievous; wily. / (having a positive connotation) Dexterous in performing an action, so as to escape notice / Done with, and marked by, artful and dexterous secrecy; subtle

Slyly.

(idiomatic) Slyly, in an inconspicuous manner, so as not to be seen; secretly; stealthily.

(dated) on the sly

A diminutive of the male given name Sylvester.

Alternative spelling of slyboots

Alternative spelling of slyboots

(simile) Very sly, very cunning.

A sly or crafty person.

(Australia, dated) A place illegally selling alcohol outside of licensed hours.