When you jab someone, you poke that person. No one on the school bus wants to sit next to the kid who tends to jab people in their ribs with his sharp elbows.
You might accidentally jab a man on the elevator with your umbrella, or jab your annoying brother on purpose when he's talking with his mouth full at dinner. Another kind of jab is a quick, sharp punch, especially the kind you make in martial arts. The word comes from the Scottish job, "to strike, pierce, or thrust," which in turn is rooted in the Middle English jobben, "to jab or thrust."