When a person or situation is so immensely whack, that the simplicity of uttering "that is whack" does not suffice. The level of whack is so monumental, and so infuriating, that only the grand stature of "Whackasaurus Rex" can do the situation justice. A tyrannosaurus rex is a monstrous and violent dinosaur, and so the correlation makes sense when someone or thing is monstrously and violently whack.
Individuals that are classified as Whackasaurus Rex, have also been known to be perpetual users of Awkward Sauce.
You see Johnny kick that little girl in the stomach?
Yea, Whackasaurus Rex, man...