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The effect of the human aging process that causes the lower part of the buttocks to "deflate", thus resulting in an upsidedown triangle shape when viewed directly. Usually more pronounced in people with high waistlines.
"Triangle Ass is a horrible disease."

"Won't you please help prevent Triangle Ass."

"Holy balls, did you see that wicked case of Triangle Ass?"
by McCrapweasel August 27, 2009