Although it's hard to prove, I suspect this may be a composite of the Mississippi Valley Siouan */he/ 'be in a place; locative be' that we find compounded in the positional auxiliaries (virtually all of them), niNk-he, thaN-he, adhiN-he, k-he. So it would be /op/ (ob) + /he/ 'be with in a location'. At any rate, it's a nice idea and it fits what we see. bob