An Irish oath or exclamation used for emphasis or surprise; a variant of 'begorrah.'
From Irish 'be Dhé ad' or 'be Dhía ort,' meaning 'may God be with you' or literally 'God with you.' It evolved into an exclamatory phrase in Irish English, becoming a stereotypical marker of Irish speech in literature.
Bedad is a great example of how languages in contact borrow entire phrases and turn them into new words—Irish English speakers incorporated this Irish blessing into their English speech as an exclamation, and it became so associated with Irish characters in literature that you'll see it in old novels as a comic stereotype.
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