I'm reading a book. How can I rephrase it? "But this bank of British Columbia thing was doing his head in." I stumbled upon a phrase in the end of the sentence.

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"was annoying him a lot."

- "Having no money at the end of the month does my head in."

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Doing his head in means making him annoyed or confusing  aggravating him  etc dependent on the context.

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"Bank of British Columbia" Always capitalize the important words in titles.