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1.
A lumpy butt caused from bathroom cleaning chemicals. Specifically happens to Mexicans because their butt DNA mutates and causes lumpy butt.
Example for "Sad Mexican Butt":

Kyle: "Yo what happened to that girls butt?

Matt: "Dude you know she cleans bathrooms at night, she has that sad Mexican butt from the chemicals."
by MGM8000 December 23, 2013