Refers to the waist or the area around the lower back. Often used in contexts of posture, movement, or physical description.
彼は小腰をかがめて挨拶した。
He bowed slightly from the waist in greeting.
小腰のあたりに痛みを感じる。
I feel pain around my lower back.
Compound of 小 (small, slight) and 腰 (waist). The exact historical derivation is uncertain, but the word likely refers to a slight or modest movement of the waist, or simply the waist itself.