I can't claim credit for this one, I picked it up on mailing list for Everway years ago, might even be Jonathan Tweet who came up with it. But I've been saving it in my bag of tricks for years and years, waiting for a chance to use it.
Amongst the people of this particular culture has arisen a long-standing tradition that seems very strange to outsiders. People have odd and disturbing names: Poison, Treachery, Despair, Infection, or Bankruptcy, for instance. It turns out that the people here believe these things to be brought by spirits of the same name: if you get poisoned, it is because you were visited by the Poison Spirit. And surely, the Poison Spirit is less likely to harm someone who has been named for him! Children are always named for some malignant spirit which the parents fear might otherwise afflict the child, as a protection against them.
(It might be even more cool if this turns out to be true -- that anyone named Poison is, in fact, immune to poison. Just need to ensure no one can get named Death, or Badness, or something else too vague or potent.)