A flap that covers an entrance or forms a rooflike
extension for a tent or the canopy of a vehicleHmmm, I'd always thought a "flysheet" was so-named because it kept of flies
. Which wouldn't be a very healthy association in the current context.
Now I can see that what most of the pertinent definitions have in common is that one bit of material/cloth lies (or flies)
over other bits of material/cloth.
So thank you, I've learned something today