Mattapan is a neighborhood in Boston, Massachusetts. Historically and for legal processes a section of Dorchester, Mattapan became a part of Boston when Dorchester was annexed in 1870. Mattapan is the original Native American name for the Dorchester area, possibly meaning "a place to sit." At the 2010 census, it had a population of 36,480, with the majā¦Mattapan is a neighborhood in Boston, Massachusetts. Historically and for legal processes a section of Dorchester, Mattapan became a part of Boston when Dorchester was annexed in 1870. Mattapan is the original Native American name for the Dorchester area, possibly meaning "a place to sit." At the 2010 census, it had a population of 36,480, with the majority of its population immigrants.