I read it here a few years ago too. Born and raised in the Midwest, lived here 60+ of my years but I have never heard it. I think because it’s a regionalism that is in Ohio, Illinois and Indiana which is really the Mideast of America. It’s not the Midwest until you’re west of the Mississippi. Pennsylvania and Ohio calling themselves the Midwest is just silly.
….Thank you! I had heard about this before, but had forgotten about it until I got your message. People in what linguists call the West Midland region do sometimes call green peppers “mangoes” or “mango peppers.” It seems to be most concentrated in Illinois, Indiana, and, like where our caller is from, Ohio.Although it’s occasionally heard in other nearby states too.
The story is that long ago, mangoes seems to have been a term more generally used for fruits that were pickled. And yes, as an aside, peppers are technically fruits, not vegetables.
One theory is that when mangos were first imported from India, they had to be pickled to make the journey, and by association, some people started using the term “mango” to refer to any fruit that was pickled.
A different story uncovered by Indianapolis Star food writer Donna Segal is that in England in the 1700s, “there was a demand for Indian-style pickles like fruit mangos stuffed with spices and kept in a vinegar brine.” But at the time, mangoes had to be grown under glass in England so they weren’t widely available, and cooks began using green bell peppers instead….
From quickanddirtytips website.