The highly likely occurrence that a woman has so much beef chops that she can wrap her pussy around a building. Beefosaurs are commonly morbidly obese. They usually are found at Burger King. Their mating call frightens skinny children into becoming Beefosaurs, and invented Obese Pornography.
A disproportionately large group of muscles on the body. Usually grotesque. Imagine a lumpy, laundry bag of meat in human form; that's a beefsac.
Can also be spelled 'beefsack'