So we've agreed what UFO means, but to state that all sightings are of human (this planets) origin seems a bit much to me.
This is true, but again, there's not proof, in that area, yes UFOs can be compared to GOD, because we don't know.
You stated that they (UFO's) are not of alien origin because there is no proof, what would you deem as proof, and is only considered proof if you accept it?
No, proofs is when something is scientifically proven, or relatively undeniable, it is a truth. It doesn't matter if you accept it or not, it is proven nonetheless. For example, it is true that oranges are orange, doesn't matter if I'm colourblind and see them blue (in which case I might not accept that they are orange), but it is proven that they are orange, there is proof because we know the wavelength at which the color shows.
What about the hundreds of cave paintings from many parts of the world that seem to depict the same creature. Maybe that Roswel thing?
When you say that a UFO sighting isn't necessarily proof aliens exist, given that many UFO sightings are fake many of them could be real, so they might well be proof that aliens exist.
Again, UFOs are not a proof of aliens because there are so many other possibilities, it is an hypothesis, and a valid one, but it is not a proof.