Harebrain

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

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

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

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."

" They commonly call the most harebrain blood-suckers, strongest thieves, the most desperate villains, treacherous rogues, inhuman murderers, rash, cruel and dissolute caitiffs, courageous and generous spirits, heroical and worthy captains, brave men at arms, valiant and renowned soldiers, possessed with a brute persuasion of false honour," as Pontus Huter in his Burgundian history complains.

I suppose they also handed on to me a harebrain humour, which it has been my chief delight to indulge.

It is a harebrain idea but it could be done.