Charles Ellicott Commentary 2 Samuel 20:8

"When they were at the great stone which is in Gibeon, Amasa came to meet them. And Joab was girded with his apparel of war that he had put on, and thereon was a girdle with a sword fastened upon his loins in the sheath thereof; and as he went forth it fell out." — 2 Samuel 20:8 (ASV)

Went before. —Translate, met. (Compare Note on 2 Samuel 19:6.)

As he went forth it fell out. —The object of this verse is to explain how Joab, in consequence of the arrangement of his dress, was able to stab Amasa without his purpose being suspected. He had a girdle bound round his military coat, and in this he had stuck a dagger so arranged that it might fall out as he advanced. He then picked this up naturally in his left hand, and stretching out his right hand to greet Amasa, his movements gave rise to no suspicion.