Tighty-whiteys are basicially white underwear in general, whether they are boxers or briefs. When a person sags his pants, he waddles like a duck when he walks.

And I'm afraid that if Justin Bieber gets too close to Miley Cyrus, she will drag him down with her, causing him to spiral out of control as well, but that's another topic.

And he pretty much sags all of his pants, except for the occasions when he dresses up formally and wears a suit and tie.