SWAYBACKED
dipped, lordotic, swayback, swaybacked
(adjective) having abnormal sagging of the spine (especially in horses)
Source: WordNet® 3.1
Etymology
Adjective
swaybacked (comparative more swaybacked, superlative most swaybacked)
Having a sagged back.
Having sagged or hollow surface.
(figuratively) Old.
Source: Wiktionary
Sway"-backed`, a.
Definition: Having the back hollow or sagged, whether naturally or as the
result of injury or weakness; -- said of horses and other animals.
Source: Webster’s Unabridged Dictionary 1913 Edition