Definitions of jab:

• noun:   a quick short straight punch
• noun:   a sharp hand gesture (resembling a blow)

  Example: "He warned me with a jab with his finger"

• noun:   the act of touching someone suddenly with your finger or elbow
• verb:   poke or thrust abruptly

  Example: "He jabbed his finger into her ribs"

• verb:   strike or punch quick and short blows
• verb:   stab or pierce

  Example: "He jabbed the piece of meat with his pocket knife"