rash

The adjective comes from Middle English rash/rasch ("hasty, headstrong"), probably from Old English *ræsċ, traced back through Proto-Germanic *raskaz to a Proto-Indo-European root meaning "to run, roll." The skin-eruption noun is a separate word, probably from Old French rasche ("skin eruption, scabies"), from a verb meaning "to scrape," ultimately from Latin rādō ("to scrape, shave").

The Old English root behind the adjective was tied to quick, flickering motion, seen in related words meaning "to flash" or "to crackle." Over time this sense of rapid movement narrowed into the idea of acting too quickly, without thought.

The skin-condition sense of rash is a doublet of raze — both trace back to Latin rādō, "to scrape or shave," via Old French words for scraping.