If I were to be punishèd
For every pun I shed
There would be no puny shed
For my punnish head.

Strictly speaking, of course, this isn’t a poem–it was merely an apparently spontaneous reply (but how many “spontaneous” remarks have been thought of and prepared in advance?)

The story was told in the following way:– “Sir,” said Johnson, “I hate a pun. A man who would perpetrate a pun would have little hesitation in picking a pocket.” Upon this Boswell hinted that his “illustrious” friend’s dislike to this species of small wit might arise from his inability to play upon words. “Sir,” roared Johnson, “if I were punishèd for every pun I shed, there would not be left a puny shed of my punnish head.”

The moral of the story was presumably for Boswell and others to guard their possessions when Doctor Johnson was around…

“statue of Samuel Johnson outside St Clement Danes Church” by ell brown is licensed under CC BY 2.0