2 Samuel 13:2 (BSB)

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.

From 2 Samuel 13. Also in the ESV.

Commentary on 2 Samuel 13:2

• Matthew Henry (Presbyterian), Matthew Henry's Complete Commentary on 2 Samuel 13:1-20: We have here a particular account of the abominable wickedness of Amnon in ravishing his sister, a subject not fit to be enlarged upon nor indeed to be mentioned without blushing, that ever any man should be so vile, especially that a son of David should be so.
• John Gill (Reformed Baptist), Exposition of the Old and New Testaments on 2 Samuel 13:2: And Amnon was so vexed,.... Distressed, straitened, and perplexed in his mind through unruly and unbridled lusts that raged in him: that he fell sick for his sister Tamar; as Antiochus son of Seleucus did for his mother in law Stratonice, who, to cure him of it, was delivered to him by his father (s): for she was a virgin; and so kept very recluse...
• Jamieson-Fausset-Brown (Reformed), Commentary Critical and Explanatory on the Whole Bible on 2 Samuel 13:2: for she was a virgin--Unmarried daughters were kept in close seclusion from the company of men; no strangers, nor even their relatives of the other sex, being permitted to see them without the presence of witnesses. Of course, Amnon must have seen Tamar, for he had conceived a violent passion for her, which, though forbidden by the law (Lev 18:11), yet with the sanction of...
• Geneva Bible Notes (Reformed), Geneva Bible Study Notes on 2 Samuel 13:2: And Amnon was so vexed, that he fell sick for his sister Tamar; for she [was] a virgin; and Amnon thought it hard for him to do any thing to her. (b) And therefore kept her father's house, as virgins were accustomed.