Where does the A Priori Come From?

The claim that there is such a thing (I use the word advisedly) as a priori knowledge has always puzzled me.  It seems clear that before we come into existence, we have no kind of knowledge at all, or that if we had any knowledge we have no knowledge of it, as it were.  It would seem to follow that what we know is then in a very basic sense contingent on our existing.  We exist, therefore we know.  Because we exist, we do--eat, sleep, think; we interact with others and the world.  We're not dormant.  Much as some of us may want to be, we're not mere observers of our environment, wax on which things make an impression.  We learn by experiencing.

All unmarried men are bachelors, true.  But, we don't know what it is to be married, or what men are for that matter, a priori.  We come to do so by living.  Men who are not married are called "bachelors" in the English language. Language is something we come to know and use by living.  We similarly come to know that X is X, and Y is not X, by living.  We encounter X and Y and recognize they're different.  There seems to me to be no question of this.  It is apparent that life is a condition precedent (as we lawyers say) to knowing anything.

Presumably, the proponents of a priori knowledge are quite aware of this very mundane fact.  What, then, do they mean by maintaining that we have a priori knowledge?

Can it be that what they mean is simply that we humans have certain characteristics or features which, when we interact with others and our environment, have certain results?  We have a brain, for example, and the brain functions in a particular way, and that because it functions in a particular way when we interact with others and the world we do things like act, and think, and feel as we do?

If that's what they mean, though, just what is it that is a priori?  Are we a priori, then?  Is the a priori knowledge just a description of a kind of thinking, or knowledge, we come to possess through living?