I'm francophone, I do my best to speak English.
What's a stupid question !
You asked "What's a stupid question ! "
Any question you already know the answer to and are only asking to validate your suspicion(s) of lying, infidelity, or anything else along those lines. These kinds of questions are commonly found in relationships or in the workplace.
A question you won't like the honest answer (am I fat, am I satisfying as a sexual partner, do I fulfill your needs, etc.). You'll occasionally run across someone who will not spare your feelings and give the honest answer.
A stupid question is one where both parties (the one asking and the one answering) will not gain any valuable insight. To further this, that means the one answering the question doesn't know the answer and neither party takes the time to research an answer. Unanswered questions can have their feelings hurt also.
A rhetorical question can be a stupid question if it doesn't continue a meaningful conversation.
Any question that can be misconstrued as racist/sexist (including generalized questions that refer to an entire race/gender as a whole), or discriminating in any way.
Incomplete questions that are vague or don't have enough description to warrant a concise answer.
I'm sure that this is a short list but I feel that it encompasses most types of stupid questions.