Asking the question "What happened before the Big Bang" is legitimate in the context that it is questioning the assumption of a Big Bang to begin with. If we say there is nothing outside the configuration space we have just proclaimed there is something called space; meaning there is an object with a boundary. We cannot have a boundary without something outside that boundary since that what defines the boundary to begin with. If we say there is no boundary to space then there is no expansion of space to begin with since it has no boundary and has always been spatially infinite. If we say there is a timeless mathematical object with time existing within it then we have just contradicted ourselves by saying it has no time yet it has it itself. By calling it a timeless mathematical object we are saying it doesn't exist except in our concept of it and since we are conceiving of it now it does exist since how could it not if we are able to conceive of it, and that conception has not changed over billions of years and since no change equals no time it is eternal. And if there is no t outside of it then there can be not expansion of it since it has no boundary, hence any notion of the Big Bang being an expansion of time and space remains such as that, a notion - without real physical reality; There was no beginning of say 14 billion, 18 billion years ago...it has always been and always will be.

It's called a loaded question. http://www.fallacyfiles.org/loadques.html

I invented a logic that can deal with questions and answers. It allows one to formalize questions with an adequately expanded predicate logic. Here's a formalization of the question:

(∃x)(∃t1)(∃t2)(BBHappenedAt[t1]∧HappenedAt[x,t2]∧Before[t2,t1])∧(x=?)

English: There is a thing, x, there are two points of time, t1 and t2, big bang happened at time t1, and x happened at time t2, and t2 is before t1, and what is x?

But if the empirical claim holds, which it does AFAIK, that BB was the first event, i.e. no prior events, then the question is false. Whatever has a false implication is itself false, or whatever is inconsistent with a truth is false.

I know a lot of physics students and some of them teach high school physics for money, and I asked them how they deal with the question. One of them said that he just gives them an analogy, it goes like this:

What is north of the north pole?

Formalized:

(∃x)(IsNorthOf[x,n])∧(x=?)

n = north pole

EN: There is a location such that it is north of the north pole, and what is it?

but we know that there is nothing north of the north pole. Because per definition it is the northernmost spot, i.e.:

(∃x)¬(∃y)(IsNorthOf[y,x]∧x=n)

EN: There is a location, x, and there isn't a location, y, such that y is north of x, and x is identical to the north pole. That is the definition formally speaking.

The question then is similarly false, because it has a false implication, or equivalently, is inconsistent with a truth (in this case a necessary truth, not a contingent).

Hope this helps with similar questions, e.g. "Why is there something rather than nothing?" (implying there is a reason/explanation, which I see no reason to accept).