by the sly en · ADV
Meanings
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(dated, not-comparable) on the sly
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."
1859, George Eliot, The Lifted Veil: