To come out in is to develop a rash or spots on the skin. Come out in a rash.
It takes in before whatever appears, which is the fixed shape of the phrase. To come out in a rash is also used figuratively for a strong reaction to an idea.
1phrasecome out inTo develop a rash or spots on the skin.
“He would come out in hives within minutes of eating shellfish.”
come out in is a phrase.
Practised in Phrasal Verbs