A mongo is a woman with a hideous body but nice face.
A beefhead is the opposite, being blessed with the body of a supermodel, but the features of a disabled gorilla after a particularly nasty car crash in which the airbag failed to deploy.
A combination of the two, mongo beefhead, is obvious.
Beefheads are the worst, because they can look nice from the back. Which can prove embarassing should you point her out to your friends, only for them to laugh when she turns round and they see her deformed visage.

See also Tip Drill

When you eat dirt for the first time and leave your family to build a shack in the woods