Silly Billy British informal noun
Used to refer to a stupid person or say that someone is behaving in a silly way, especially a child.
He is such a Silly Billy that he has given me the keys to his car.
Did you see that Silly Billy across the road? He was acting foolishly
The plural form of the phrase is "silly billies".
The phrase was first recorded in 1840–1850.
To say, or believe something or someone that seems completely crazy, delusional, or stupid
You spent 1000$ on a new pair of shoes! It is insanely crazy. You need your head examined.