Some of them probably are just repeating something they heard in a song, or in a movie, or on TV, hoping somebody else will recognize it and say, "I know where that's from! You're cool!" That's something kids do - play the "cool" lottery. And the more threatening or otherwise taboo or "edgy" it is, the more cool-points are awarded. I do still think that unless there's a direct, physical threat of violence, you should back away... but for how long? I don't know. It does creep.