That's another old Southernism, as well, meaning Why. It's usually used in a sense of wonderment, as in Why On Earth? And depending on the stress on the syllables and the facial expression of the user, might be equal to the daunting phrase Why on God's Green Earth . . ., which will stop a bear in his tracks.
And I've got a bit of a wonderin' going on, myself, and I can't remember anything I posted or said or linked that might have caused it, but a GREAT number of my drop-ins lately have come in via this post:
I cannot for the life of me imagine why. Or what they followed to get there. Their landing spot is directly there, without going through the current post---it's like there's a little X for a helipad right there, and in they drop, like a Lifeline helicopter. Does anyone know what I did, or how that happened? If any new folks happen this way because of that, I'd sure like a comment to tell me how you arrived here.
And you're very welcome, whatever the route. I hope you'll follow it again.