Sentence:  It resembles the perch (unfortunately for itself) in having a very long and prickly fin on its back, advantage of which is taken by the boys about Windsor, who are very fond of 'plugging a <WSD>pope</WSD>.' This operation consists in fixing a bung in the sharp spines on the poor <WSD>pope's</WSD> back fin, and then throwing him into the water. 
Meaning of the word marked with <WSD> tags:  (fish) The ruffe, a small Eurasian freshwater fish ((ver=161201)); others of its genus.