Green’s Dictionary of Slang

used of a horse that persists in falling to its knees.

1860		Hotten Dict. of Modern Sl. etc. (2nd edn).
1864, 1867, 1870		Hotten Sl. Dict.
1873		Sl. Dict.
1895		Crowe Aus. Sl. Dict. 23: Devotional Habits, said of horses weak in the knees.