What does 2 Samuel 20:8 mean?

When they got to the big stone at Gibeon, Amasa met them there. Joab was wearing his military uniform, with a sword strapped to his waist. As he stepped forward, the sword slipped out. 2 Samuel 20:8 - Modern Text Bible

(Amasa catches up with the troops at Gibeon. Joab, dressed for battle, approaches him, setting up a tense encounter.)