What does it mean if you cough up blood after you are stabbed?

Injury: Usually coughing up blood after a stab wound to the chest is related to injury to the bronchial (airway) tree. Other possibilities would include rupture to a vessel (there are many in the chest) or damage to the heart (although this usually causes much more severe symptoms and is typically not associated with coughing up blood. A chest xray is imperative if you cough blood up after wound.

Your Bleeding: If you cough up blood, you are obviously bleeding somewhere in the respiratory tract or the esophagus or stomach. It may not have anything to do with being stabbed if you were stabbed in the leg. If you are stabbed in the chest, you should be in the emergency room getting treatment.