Charles Ellicott Commentary 2 Samuel 13:5

"And Jonadab said unto him, Lay thee down on thy bed, and feign thyself sick: and when thy father cometh to see thee, say unto him, Let my sister Tamar come, I pray thee, and give me bread to eat, and dress the food in my sight, that I may see it, and eat it from her hand." — 2 Samuel 13:5 (ASV)

Make yourself sick. —Rather, Feign yourself sick. It has already been mentioned in 2 Samuel 13:2 that Amnon “fell sick.” That was the real pining of ungoverned and ungratified passion; this was a crafty feigning of sickness. Yet the miserable condition to which Amnon was brought by the former would give color and plausibility to the latter.