A word or phrase which, when uttered aloud, is sure to cause offense or a fight, especially a vulgar ethnic epithet spoken to a member of the group to which it refers.
"Then he said the Magic Word and the fun started -- five minutes later that bar was wrecked and someone's head had been pushed into the deep fat fryer. We left just ahead of the cops, the firemen, and the ambulance."
Most moms say this as a way to get their kids to mind their Ps and Qs. But really the magic word(s) are "HEY THIS IS LIBRARY" It means "mind your Ps and Qs" or "Say please" to get children to say their manners.