David Ortiz received the nickname "Big Papi" because of his tendency to call people "papi" or "mami" when he couldn't remember their names. He received the nickname during his time playing for the Boston Red Sox.
While he was a young player for the Minnesota Twins, he originally received the nickname the Big O. This nickname was given to him by his team mates in Minnesota. When he joined the Boston Red Sox, his nicknamed changed to Big Papi. Papi is a Spanish slang term used to refer to attractive or sexy males, explains Cosmopolitan.