just a guess: https://duckduckgo.com/?t=ffsb&q=autumn+purple+ash+leaf&iax=images&ia=images

of course.. the mother plant has compound leaves... and you probably would have spotted such ...

when i had a couple.. fall color was extremely variable .... i destroyed them all when EAB hit ...

ken

I could like to suggest nyssa sylvatica?

This leaf is entire so not an ash. Nyssa sylvatica looks right.

I think Ken meant it could be a single leaflet from an ash. He mentions that it’s compound. However, 5 x 6 inches seems large for an ash leaflet.

My first thought was black gum but of the thousands of black gums I have seen over the years, dont recall ever seeing the purple underside. Could it be a variation ?

If it’s an autumn leaf post turning couldn’t it be red both sides?

purple is often red with a wax coating ... does it rub off ... ken

Leaf base is eared on the one side and nearly so on the other. If this is not abnormal for the species it will be significant.

Purple is not waxy and will not rub off. Good eye on the leaf base. I will keep an eye out for another fallen leaf and compare. These leaves tend to show up after strong winds.

Source tree has to be around there somewhere.