It's southern slanf for wallow - as in a Hog Wallow - a mud-hole dug out or yallowed out by a hog. Used to describe a hole that has been enlarged unintentionally by being a loose fit - wallered out
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The hogs on my uncle's farm in the red clay flatlands of Georgia had turned all of their pen into a gooey smelly waller, but Grandpa's on the mountain hadn't. Different soil and better drainage, I reckon.
They have some different language and pronunciation there that they loved to tease us NH Yankees with. A peCAHN is a nut, a pee-can is a urinal.
I could like living there if it wasn't so hot and humid in summer. It's 57F in here and I've acclimated to it.