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A filthy boner is one that, in the course of festivities, has become covered in filth.
Rupert woke up with a boner: one that was covered in cum, poo, vag juice, taco sauce, pubes, and dog slobber.

"Wow," he thought as he looked up to see Cindy's panties hanging from a floor lamp, "that must have been a great party."

"I have a filthy boner!"
by scodder April 24, 2010