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Phrase(s): break out in a rash
[for the skin] to erupt with a rash. (See also break out in a cold sweat; break out (with a rash).) • I knew Dan had the chicken pox, because he broke out in a rash and had a dry cough. • The baby breaks out in a rash all the time.
McGrawhill's American Idioms And Phrasal Verbs