Harebrain

...rash harebrain treating matters that require age and gravity;

Source: wiktionary

At the moment she was watching something behind my back, undoubtedly that harebrain who was hanging over the street.

Source: wiktionary

He smiled uncontrollably while driving; and then, thought that other drivers looking at his face would conclude there goes an unhinged harebrain.

Source: wiktionary

I mean it (saith the king) by that same harebrain, wild-fellow, my subject, the Earl of Suffolk, who is protected in your country, and begins to play the fool, when all others are weary of it."

Source: wiktionary