Used to express surprise.
[Alteration of VOW2.]
Our Living Language New Englanders sometimes express surprise by saying, "Well, I vum!" This odd-sounding word is in fact an alteration of the verb vow that goes back to the days of the American Revolution. It is also heard simply as "Vum!" or as a sort of past participle: "I'll be vummed!" A Southern equivalent is swan or swanny, also meaning "swear": "Now, I swanny!" According to the Oxford English Dictionary, the word swanny derives from the dialect of the north of England: I s' wan ye, "I shall warrant ye" (that is, "I shall guarantee you").
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