A long time ago, in America, there was a beehive in the middle of a forest. Every day, as worker bees do, they would go out into their fields, gather pollen from the flowers, and bring it back to make honey.
The bees had a problem, though, because every so often an intruder would come around, such as a bear who wanted the honey, or kids who thought it'd be fun to throw rocks at the hive. Finally, the bees got tired of it. Being the intelligent bees that they are, they built an alarm system for the hive.
They built it such that one bee pulls a lever, which triggers the alarm that the bees will hear from the fields, and then the bees can come back to protect their home.
There was one bee who was exclusively assigned that job, and he was aptly named the "Lever Bee."
His job was to watch for potential adversaries, and pull the lever to raise the alarm. Now clearly, the safety of the hive depends on this one Lever Bee. So it stands to reason that he has to be constantly ready and on the alert so that he can do his job.
And that, friends, is why people say, "I'm as ready as a Lever Bee."