A practical sex joke similar to a Dirty Sanchez
and such. To prepare for a Dirty Pinto one must eat a shit-load of pinto beans before getting a blowjob. Then right before ejaculating, turn around and shit in the females face, effectively covering her in a "shit-load" of Pinto Beans.
This maneuver requires the utmost speed and skill and one must be able to turn as fast as a ninja.
Todd gave this slutty whore a "Dirty Pinto", it took her an hour to wash her face and she still smelled like shit.