If someone is rash or does rash things, they act without thinking carefully first, and therefore make mistakes or behave foolishly.

A rash is an area of red spots that appears on your skin when you are ill or have a bad reaction to something that you have eaten or touched.

If you talk about a rash of events or things, you mean a large number of unpleasant events or undesirable things, which have happened or appeared within a short period of time.