A ridiculously large penis.. usually meant to describe a black man's penis but not necessarily.
The term was further popularized by the black pornographic actor / director for West Coast Productions studio who goes by the screen name "Mandingo", for obvious reasons. His cum shots are epic yet disturbing.. and can be compared to the visually equivalent situation in which a woman gets a gallon (or 2) of glue poured all over her face.
Have you ever seen a mandingo? You don't know what that is? Want to find out?