Wikipedia
Jonadab is a character in the Hebrew Bible, appearing in 2 Samuel 13. He is described in verse 3 as the son of Shimeah, who was the brother of David, making Jonadab a cousin to Amnon as well as his friend. He is called "very wise" (ḥākām mĕ'ōd), usually translated as "very shrewd" ( NIV) or "very crafty" ( ESV).
2 Samuel 13 describes how Amnon wanted to have Tamar, despite the fact that she was his half-sister. Jonadab advised Amnon to pretend to be sick, and then ask David to send Tamar to him to make him some food. Amnon followed Jonadab's advice, and ended up raping Tamar.
Jonadab appears again at the end of the chapter, when he tells David of Absalom's grudge against Amnon. Pamela Tamarkin Reis notes that he was "privy to the confidence of both brothers", and suggests that he "told Absalom about Amnon's scheme simply because he was a busybody, stirring his spoon in every pot."
Keith Bodner suggests that "among the numerous minor characters in 2 Samuel 11–19, Jonadab remains among the most enigmatic, and his malignancy marks a key juncture in the story."