If you are impulsive, you do the thing first and think later. A feeling arrives, and you move — you buy the ticket, send the message, say yes — before you have looked at the trouble that might follow. We use it of people (an impulsive friend) and of acts (an impulsive purchase, an impulsive decision). It is a common word, not fancy. It is not always a scolding. People sometimes say it with a smile: impulsive can sound alive and warm, not only foolish. But in careful advice it is a warning: you did not weigh the danger. It is different from imprudent, though they sit near each other. Impulsive points at speed and feeling; imprudent points at poor judgment. You can be slow and still imprudent. You can be impulsive and lucky. In more technical English, impulsive can also describe a force that hits for a very short time, then is gone. That is a different, narrower use. Though actually, after a bad result, people often call the same act both impulsive and imprudent.