Swayback (noun)

1. A condition in which the spine of an animal, especially a horse, is curved in a concave shape, causing the back to appear to be sagging.
2. A condition in which a person's back is curved in a concave shape, often caused by poor posture or weak abdominal muscles.

Origin:

From the verb sway + back.

Examples:

1. The horse had a pronounced swayback, making it unsuitable for riding.
2. The cow had a swayback, it was weak and would have trouble walking.
3. The swayback chair was uncomfortable to sit in.
4. The swayback posture was caused by a weak core.
5. The swayback condition was caused by a congenital disorder.