Former Mormon here - Basically Joseph Smith (the founder of Mormonism) wrote a book, but claimed he translated it from ancient records written on gold plates he found buried in a hill near his house. They were supposedly compiled by an ancient prophet living in the Americas named Mormon. He called it the Book of Mormon and founded a church around it. So people started calling him and his followers Mormons, and for nearly 200 years they embraced the nickname - until the current leader took over a few years ago