Superannuated: (adjective), incapacitated or disqualified for active duty by advanced age

“Now that the small field beyond the orchard had been set aside for barley, it was rumored that a corner of the large pasture was to be fenced off and turned into a grazing-ground for superannuated animals.” (George Orwell’s Animal Farm)

I was helping my brother with his homework and stumbled across this word that I hadn’t known either. I was confused because I couldn’t exactly figure out the definition from the surrounding text. But I figured the words roots were something along the lines of “super” and “annual” meaning old. After the definition I now know that a corner of the pasture was going to be turned into a grazing field for old animals that can no longer serve the farm in any meaningful way, because of their old age.