falling suddenly; already fallen
Rare literary noun. Most often seen in the set phrase 既倒に懲りる (learning from a past failure) or in classical descriptions of something that has already collapsed.
既倒 is an old expression referring to something that has already fallen.
彼は既倒に懲りて、同じ失敗を繰り返さなかった。
Learning from his past failure, he did not repeat the same mistake.