To reject a candidate for literary honours because he is not up to the required mark. The rejected candidate is said to be plucked.
When degrees are conferred the name of each person is read out before he is presented to the Vice-Chancellor. The proctor used at one time to walk once up and down the room, and anyone who objected to the degree being conferred might signify his dissent by plucking or twitching the proctor's gown. This was occasionally done by tradesmen to whom the candidate was in debt; but now all persons likely to be objected to, either by tradesmen or examiners, know it beforehand, and keep away. They are virtually plucked, but not really so.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894