NOTE: "what the butt" / WTB carries an alternate definition by the simple insertion of a comma, as in: "What, the butt?!" Or, "What, the booty?!" The initialized version is universally accepted as a code phrase to express, "Are you signaling me to look at that stranger's rear end?"
At the Grand Canyon, you accidentally step from the rim and into the abyss. As you're falling to your death, you scream, "What the butt?!"
Your favorite anti-antiperspirant is sold-out. With a sigh, you say, "What the butt?!" to yourself.
And for the ALTERNATE version: While walking through the mall, your buddy elbows you, then nods his head toward a woman walking just ahead. She has funky hair, but also an ass that is larger-than-life. You ask your friend, "WTB?" to mean, "What, the butt?" just to be sure he's not referring to the hair-do.