This expression came up in another thread, as it does from tine to time, and it suddenly occurred to me to wonder where the hell the phrase comes from.

More to the point, how has it come to be used to mean doing something which is very easy, or picking on targets impossible to miss? For in reality "shooting at fish in a barrel" would surely be remarkably hard to do, and you'd very likely end up killing yourself or someone else.

With a wooden bucket you'd shoot straight through, with a metal bucket the bullet would likely bounce out agauin - and even if you used a pea-shooter, hitting a fish would be extraordinarily difficult, given the way water refracts light.

Normally, when curiosity gets awoken by a phrase in this manner, it's an easy matter to solve the riddle through Brewer's Dictionary of Phrase or Fable, or via Google. But this time, no luck. Plenty of people using the phrase, but no indication could I find as to where it comes from, and how it can possibly make any kind of sense.

So can anybody out there throw any light on it?