sacrococcygeus dorsalis

noun

sa·​cro·​coc·​cy·​geus dor·​sa·​lis -käk-ˈsij-ē-əs-ˌdȯr-ˈsā-ləs 

: an inconstant muscle that sometimes extends from the dorsal part of the sacrum to the coccyx

called also posterior sacrococcygeal muscle