2 Samuel 20:8

the great stone which is in Gibeon Some isolated rock, well known as a landmark.

went before them Lit. came before them, i.e. met them, apparently as he was returning to Jerusalem, after raising an army in Benjamin as well as in Judah.

And Joab's garment, &c. This description of Joab's dress is intended to explain how he contrived to stab Amasa without his design being suspected. In the girdle which he wore over his military dress was stuck a dagger. As he met Amasa he contrived to let this fall out of its sheath on the ground. He picked it up in his left hand, and the movement being apparently purely accidental, excited no alarm in Amasa's mind.

as he went forth it fell out As he went out to meet Amasa. But it does not appear whenceJoab went out, and the words may also be rendered, and it(the sheath) came out(from the girdle), and it(the sword) fell down. The Sept. has a simpler reading, involving a slight change in the Heb. text: and the sword came out and fell down.