When you do something without thinking it through carefully, you act impetuously. There's a strong possibility you'll regret impetuously volunteering to give your friend's pet tarantula a bath.
While doing things impetuously can occasionally have good outcomes, this adverb is generally used in a negative way, for actions taken in the heat of anger or impatience. A toddler who acts impetuously, suddenly throwing his lunch on the floor, can be frustrating. And a government leader who acts impetuously, abruptly starting a war, can be dangerous. The Latin root is impetus, "attack."