An Izaak Wallon. One devoted to “the gentle craft” of angling. Izaak Walton wrote a book called The Complete Angler, or Contemplative Man's Recreation. (1655.)
“Gentle” is a pun. Gentles are the larvae of flesh-flies used as bait in angling.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894