Hi porfavor, welcome to AWADtalk!

wwh is right - that opening in the trousers was called a fly long before the zipper came along. In fact, it was called "flies" - eg, "your flies are undone" - and I suspect, thought I'm guessing here, that the expression arose from the fact that the flap was done up with buttons, which perhaps were equated with actual flies (of the insect variety) because of their size....

Anyone like to correct me on this? is it just me being wildly creative or is there a grain of truth in what I say?

Let us go in peace to love and serve the board.