This professor teaches with life; he never just trots out quotations.
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Originally mocking Southern Tang scholar 彭利用, who answered every question by tossing in obscure classical quotes. Recorded in 《南唐书·彭利用传》, which says he larded even everyday talk with chopped-up book quotes, so people dubbed it 掉书袋.
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Always pejorative. Calling someone 掉书袋 means they're boring the room with their learning.