If you do something rashly, you do it too fast, without stopping to think whether it is wise. You might rashly promise to help, rashly spend money, or rashly say yes, and only later see the problem. It is a criticism, but a fairly ordinary one — not as hard as calling someone stupid. People use it in speech and in writing. Though actually, a rash act can still turn out well; the word is about the speed of the choice, not always about a disaster. A careful person may also call a brave choice rashly made, so listen for who is judging.