noun
legs and lower back; lower body
Refers to the legs and the lower back/hip area as a functional unit, often in contexts of physical strength, mobility, or health.
年を取ると足腰が弱くなる。
As you get older, your legs and lower back become weaker.
毎日の散歩で足腰を鍛えている。
I strengthen my legs and lower back with a daily walk.
noun
Figurative extension: the base or support of an organization, system, or argument. Often used with 弱い or 強い.
この計画は足腰が弱い。
This plan has a weak foundation.
組織の足腰を強くする改革が必要だ。
Reforms are needed to strengthen the organization's underpinnings.