I occasionally run across a phrase containing the words 'mad impetuous boy' in a number of movies. Probably most famous is a Pete Lorre lookalike in a Loony Tunes cartoon being slapped by Daffy Duck.
It's such a specific phrase, I wonder if there's an origin, and due to the nature of Loony Tunes cartoons being full of references to the culture of the time, I suspect the origin is not the Loony Tunes cartoon.
I googled it for some time without getting anywhere.