A rash is something that spreads like wild fire — red itchy skin or a series of unfortunate events. It can also describe an impulsive, wild decision.
As an adjective, rash has meant "quick, vigorous" since the 1300s by way of Scotland. The meaning shifted to "reckless" a few hundred years later, and can still be used that way — a "rash decision" is a sudden, not well thought out one. Rash, the noun that no one wants on their skin, came a few hundred years after that, but from the French word rache which at some point meant "ringworm." Ringworm still gives us a red, itchy rash. Fun! Rash can also mean a lot of unpleasant things happening in a short amount of time, like robberies or earthquakes.