Back in the 1800's, it literally was
used in the fishing industry to differentiate between work assignments - either
to have the job of fishing or the job of cutting the bait used to fish.
Today it is commonly used to act or not
act; to take action or alternatively, to procrastinate or stand down. A far cry from the act of fishing.
So given the choice, let's fish.
And that's it - lock, stock and barrel.