ride up
Refers generally to a condition in which one's pants, skirt, or underwear gradually slide up the legs/hips/waist, often resulting in a major wedgie if left unchecked. Can be caused by suspenders, poorly-fitting clothes, shifting positions while sitting, or exercise.
Seeing Gretchen's skirt ride up her fat cellulite-infested thighs reminded me that I had to pick up some cottage cheese at Safeway on the way home from work.
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