Acting without carefully considering the consequences is rashness. If you impulsively jump off the roof of the shed into a pile of leaves, you may regret your rashness.
Rashness is a kind of carelessness, a failure to think about how your actions might harm yourself or someone else. Your rashness can result in hurt feelings, if you blurt out every thought without considering the effects, or in making your parents mad, if you impulsively paint the living room hot pink. Rashness comes from rash, "proceeding from a lack of consideration of consequences," a Scottish word originally meaning "nimble or quick."