Green’s Dictionary of Slang

a very rare or extraordinary event.

c.1698		B.E. Dict. Canting Crew n.p.: A Chop by chance, a rare Contingence, and extraordinary or uncommon Event, out of course.
1725		New Canting Dict. n.p.: chop [...] a Job, or Booty, as, A Chop by Chance, a rare Booty, an extraordinary or uncommon Prize, out of course, when ’twas not expected.
1737, 1759, 1760, 1776		Bailey Universal Etym. Eng. Dict. [as cit. 1725].