na-adjective, noun
bad back; spineless person
Used both literally for a weak or painful lower back and figuratively for a person who lacks resolve or easily gives in. The figurative use is colloquial and somewhat derogatory.
最近、腰弱で長時間座っていられない。
Lately, my bad back makes it hard to sit for long periods.
彼は腰弱だから、交渉ではすぐに折れてしまう。
He's spineless, so he gives in quickly during negotiations.