Amnon was sick with frustration over his sister Tamar, for she was a virgin, and it seemed implausible for him to do anything to her.
Jewish Publication Society of America Version (JPS1917) - 2 Samuel 13:2 >And Amnon was so distressed that he fell sick because of his sister Tamar; for she was a virgin; and it seemed hard to Amnon to do any thing unto her.