Doth on the sly is an old-fashioned phrase that means to do something secretly or sneakily. There are many synonyms for this phrase, including sneaky, furtive, clandestine, surreptitious, and covert. These words all describe actions that are done in secret or with a hidden motive. Other synonyms for doth on the sly include underhanded, cunning, and deceitful. These words suggest a level of craftiness or manipulation that is often associated with secretive behavior. Whatever word you choose to use as a synonym for doth on the sly, it is clear that the action being taken is one that is meant to be hidden from view or done without others knowing about it.