A rash decision is one made too quickly, without enough thought about what might go wrong. When you act rashly, you rush into something before considering the consequences. A student might make a rash choice to skip studying for a big test because their friends invited them to play video games, only to regret it the next day when they struggle with questions they could have answered.

The word suggests impulsiveness and carelessness. Someone might rashly spend all their allowance on candy at the beginning of the week, leaving nothing for the book fair on Friday. A knight in a story might rashly charge into battle without a plan, putting everyone in danger. Being rash is different from being decisive: a decisive person thinks quickly but carefully, while a rash person doesn't think enough at all.

The word can also mean a skin condition where red, bumpy spots appear on your body. You might develop a rash from poison ivy, an allergic reaction, or certain illnesses. This kind of rash can appear suddenly and spread across the skin, which is actually how the “hasty decision” meaning connects to it: both spread quickly and show up without much warning. A doctor might ask if you've eaten anything new or touched something unusual to figure out what caused your rash.