Albert Barnes Commentary 2 Samuel 3:29

"let it fall upon the head of Joab, and upon all his father`s house; and let there not fail from the house of Joab one that hath an issue, or that is a leper, or that leaneth on a staff, or that falleth by the sword, or that lacketh bread." — 2 Samuel 3:29 (ASV)

The curse of David proves that Joab was not justified as a blood-avenger or goel in taking Abner’s life (2 Samuel 3:27).

That leans on a staff — Rather, a crutch. The phrase denotes one who is lame or infirm. For similar instances of hereditary disease and poverty as a punishment for great sin, see 1 Samuel 2:31–33, 1 Samuel 2:36; 2 Kings 5:27; and John 9:2.