背宍[sojishi] meat, flesh, or muscles along the spine

鯔背[inase] gallant
違背[ihai] violation
棄背[kihai] giving up and turning back
向背[kouhai] one's attitude
上背[uwazei] stature

側背[sokuhai] flank
団栗の背比べ[dongurinoseikurabe] having no outstanding characteristics
中肉中背[chuunikuchuuzei] medium build
中背[chuuzei] average height
二律背反[niritsuhaihan] antinomy