noun, noun which may take the genitive case particle 'no'
waist; hips; pelvic region; loins
Refers to the lower back and hip area, often in anatomical, medical, or descriptive contexts. Can be used as a noun or with の to modify another noun.
腰部に痛みを感じる。
I feel pain in my lower back/hip area.
Long hours of sitting work put strain on the lower back.
腰部の筋肉を鍛える運動をしています。
I do exercises to strengthen the muscles of the lower back and hips.
She went to the hospital because of lower back pain.
This belt is designed to support the lower back.
He hurt his lower back lifting a heavy load.
臀部 refers specifically to the buttocks, whereas 腰部 covers the broader lower-back and hip region.
Compound of 腰 (こし, 'waist, lower back') and 部 (ぶ, 'part, region'). The reading ようぶ uses the on'yomi of both characters, forming a technical anatomical term.