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》: '对家人稚子,下逮奴隶,言必据书史,断章破句,以代常谈,俗谓之掉书袋.'
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Always pejorative. Calling someone 掉书袋 means they're boring the room with their learning.