There's not a specific instance that brings this to mind now, just something that I've noticed for quite some time. In popular culture the terms "mature" and "adult" tend to mean the opposite of the literal, common-sense meaning, in part I suppose because of Hollywood ratings systems. But I think most would agree that when you see something labeled "for mature audiences" or "adult material," it really means something designed to appeal to the adolescent (perhaps the adolescent still lurking in all of us?) with a particularly prurient frame of mind (or glands).
Adolescents seem fascinated with sex and violence, so naturally our would-be keepers want to "protect" them from such nasty things so they label films, books and such with lots of sex and violence "adult," or "mature," even though most reasonably mature adults have probably outgrown the need for porn or quasi-porn or undiluted violence (perhaps because they have actually experienced both and the glamor is diluted (or gone in the case of actual violence) and the "forbidden fruit" syndrome no longer applies).
Do you know of other examples where words in common or popular parlance turn out to mean pretty close to the opposite of what the dictionary or common-sense meaning is? I have but can't think of them just now. Additions to the list are welcome.