Definition of "on the sly" []

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

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• "She was in the habit of sealing up the empty wine-jars, as well as those which were full, so that a jar emptied on the sly by a guzzling slave might be at once known."
• "He watched her on the sly at practices, carefully, because Shlyakhov became furious when other men looked at her."
• "Danny found the Teamster picket boss, picture pure; the man winked on the sly and handed him a pine slab with UAES — UN-AMERICAN ESCAPED SUBVERSIVES printed on reinforced cardboard at the top."