/ Thesaurus / O Synonyms of on the sly Find synonyms for: Adverb1. furtively, on the slyusage: in a furtive manner; "the soldiers were furtively crawling through the night"WordNet 3.0 Copyright © 2006 by Princeton University. All rights reserved. Related Content Daily Word Quiz: rustic Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language