by the sly

English

Adverb

by the sly (not comparable)

1. (dated) on the sly
   • 1859, George Eliot, The Lifted Veil:

     I read Plutarch, and Shakespeare, and Don Quixote by the sly, and supplied myself in that way with wandering thoughts, while my tutor was assuring me that "an improved man, as distinguished from an ignorant one, was a man who knew the reason why water ran downhill."