Refers to the side of the body (human or animal), or by extension the side of an object. Common in anatomical and everyday contexts.
馬の横腹を軽く蹴った。
I lightly kicked the horse's flank.
転んで横腹を打った。
I fell and hit my side.
Compound of 横 (yoko, 'side') and 腹 (hara, 'belly'). The reading ばら is a rendaku form of はら.