Underhand is a term used to describe an action that is done in a sneaky or deceitful manner. There are several synonyms for underhand that can be used to describe such behavior. Some of the most common synonyms include sneaky, sly, devious, deceitful, and dishonest. Other synonyms for underhand include underhanded, shady, crooked, fraudulent, and treacherous. These words can be used to describe a person or action that is done with the intent of deceiving others or achieving a hidden agenda. Regardless of the synonym used, underhand behavior is widely frowned upon and can lead to negative consequences.