To be underhand means to behave in a deceitful or secretive way. There are several synonyms that can be used to describe this kind of behavior, such as being sly, cunning, or devious. Other synonyms include being duplicitous, dishonest, underhanded, or treacherous. People who behave in an underhand way can also be described as being sneaky, sly, or tricky. They may also be referred to as being insidious, scheming, or conniving. Overall, there are plenty of synonyms that can accurately describe someone who is being underhand, and all of these words convey a sense of dishonesty and lack of trustworthiness.