Doi (or Doy)

My beloved mother, an Ozark hillbilly like me, tells me that when her grandmother needed to take a pee, her preferred euphemism for this action was "doy" (or "doi" . . . Mom never saw it in writing, so has no idea how it would have been spelled). 

I would love to know the origin of this part of my great-grandmother's idiolect. According to Mom, her grandmother was also prone to malapropisms (for example, often remarking, "It's as plain as the note on your face!") . . . so perhaps this expression was an Ozark hillbilly original, or a corruption of a regular English word? Or was it a leftover fragment from German, Scotch, Irish, or Native American "roots"? 

Has anyone ever heard of a similar expression?