Ass in a lion's skin rude
The "ass" in this case is a donkey, who is attempting to imitate a lion.
A blustering idiot
I told you that you were wrong, but you insisted that you were right. You're such an ass in a lion's skin!
(n): a fool person,
(adj): stupid, foolish, silly
Not intelligent, dumb or foolish
1. Injured people, usually soldiers whose injuries are not fatal and do not disable them.
2. Someone whose mind or body is damaged, and is still able to continue their daily lives as normal.
3. Unintelligent people.
Extremely stupid
This spoken phrase derives from Aesop's Fables.