means A skin irritation marked by red bumps, spots, or inflammation — or, as an adjective, acting hastily without thinking it through.

from Two senses braided together. The adjective 'rash,' meaning hasty or reckless, is the older one, related to Middle English and Germanic words for 'quick' or 'nimble' — a cousin of Dutch 'rasch' and German 'rasch,' both meaning 'swift.' The skin-eruption noun arrived later, likely from French 'rache' or Old French 'rasche' (an eruption or scurf), possibly tracing back to Latin 'rasus,' meaning 'scraped' — the same root that gives us 'raze' and 'razor.' So the spotty kind of rash may carry a faint echo of skin being scraped raw.