back·side 
/ˈbækˌsaɪd/ noun
plural back·sides /ˈbækˌsaɪd/ noun [count] informal : the part of the body that a person sits on : buttocks
▪ She slid down the snowy hill on her backside. [=behind, bottom]
▪ She slid down the snowy hill on her backside. [=behind, bottom]
