Skin irritations or outbreaks of red bumps or irritated skin; or hasty decisions made without careful thought.
Skin rash comes from Old French 'rache' (scabies). The figurative sense of 'rash' (hasty) comes from Old Norse 'roskr' (vigorous), which shifted to mean acting without caution. These are two different etymologies for similar-sounding words.
Heat rashes occur because sweat glands get blocked—your body is literally trying to cool itself but can't, creating the irritation—which is why tropical sailors and manual laborers historically suffered them most and why modern synthetic fabrics made them worse initially.
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