Albert Barnes Commentary 2 Samuel 13:3

"But Amnon had a friend, whose name was Jonadab, the son of Shimeah, David`s brother; and Jonadab was a very subtle man." — 2 Samuel 13:3 (ASV)

Shimeah - Called Shamma (marginal reference), was Jesse’s third son.

Subtle - Literally, wise. The word is generally used in a good sense, but here, and in Job 5:13, it means crafty.