From Longman Dictionary of Contemporary Englishon the slyon the slyinformalSECRET secretly, especially when you are doing something that you should not do They’d been seeing each other on the sly for months. → sly
Examples from the Corpus
on the sly• Dick had started drinking again on the sly.• He certainly wasn't dumping his pints on the floor on the sly.• I won't be party to your bitching yourself on the sly.• So we continued to meet on the sly.• Elmer Gantry feels guilty for his vices: seeing people on the sly and drinking.• Sethe made a dress on the sly and Halle hung his hitching rope from a nail on the wall of her cabin.• Sometimes she would give him money on the sly and he would help her with chores.• The smartest girl passes answers on the sly to her table-mates.• We try and slip him in on the sly when we think we've got them hooked. 