The Granny Pantie Waddle is when a very large woman, likely over 400# (28 stone), shakes her rear end for a show or performance. Women this large have much fat on the rear end that shakes like a magnitude 9 earthquake. The name is derived from the fact that women this large are relegated to wearing granny panties due to their size and for the flexibility that such undergarments offer.
God, look at that large girl do the granny pantie waddle. That's so hot.