a Mexican Dumptruck is using a pickup truck to dump a load of cargo similar to a dumptruck. The load is placed in the bed of the truck and the tailgate is left down. The truck is driven in reverse at a high rate of speed towards the intended "dump" point. Just before reaching the "dump" point the driver slams on the breaks causing the cargo to come flying out of the truck.
"I don't really want to unload all this firewood by hand... let's just do the ol' Mexican Dumptruck"
A Mexican Dumptruck is when one takes a Mexican's Sombrero, and shits in it so once the Mexican has put the sombrero back on, he gets covered in shit.
Will: "Bobby! What are you doing?"
Bobby shits in Diego's sombrero.
Bobby: "Puto stole my job."
Bobby Gave Diego a Mexican Dumptruck.
"Hey Jose, we need another Mexican Dumptruck load of concrete over here."