"Spavined" originally meant (for horses) suffering from a disease that causes swelling of the hock (ankle) joint and lameness. Samuel Johnson made fun of ridiculous fashions by commenting:
"Why does nobody," said our Doctor, "begin the fashion of driving six spavined horses, all spavined of the same leg? It would have a mighty pretty effect, and produce the distinction of doing something worse than the common way."
From this literal meaning came a more metaphorical one: old, decrepit, broken-down, damaged, dilapidated, or over the hill. And no longer was it applied just to horses or even only to things with legs, either.
"Adding to that the high prices and frequently spavined monorail system... people were ready to be amused." (Steve Westren, "Big budget, dull exhibits focus attention on live acts," Juggler's World: Vol. 38, No. 4, Winter 1986)
"Britain - A country which was, until the Queen Mother brought out the best in it, widely considered to be damaged and diminished beyond repair, a spavined, crime-ridden, vulgar, ignorant apology for a nation whose public services make us the laughing stock of the developed world." (Catherine Bennett, "Good Grief," The Guardian
Thursday, April 11, 2002)