To stab is to thrust or jab something sharp, the way you stab your sandwich with a toothpick or the way Brutus (and others) stab Julius Caesar in Shakespeare's play.
You can stab at the soil in your garden with a trowel or stab the steak on your plate with a fork. You might then experience a stab (or sharp feeling) of regret, wondering if you should become a vegetarian. When you "take a stab at something," you attempt it: "I decided to take a stab at opera singing." Stab comes from the Scottish stob, "to pierce."