The side or flank of the body (around the ribs/side area); often used when referring to injury or pain there.
試合中に横腹を強く打たれて悶絶した。
He was hit hard in the flank during the match and groaned in pain.
医者に横腹の痛みを詳しく説明した。
I explained the pain in my side to the doctor in detail.
相手の横腹を狙ってパスを出した。
I passed the ball aiming for the opponent's flank.