What is the origin of the phrase 'thinking about the immortality of the crab'?

Have you ever been told that? Generally used when a person is staring, as if he had lost in thought.

Whether he ask? Thinking about the immortality of the crab? or her to respond with this phrase to avoid answering to ask what do you think? But where did the phrase?
According to Wisdom's Ladder, the belief that the crab did not die was kept for many years due to the ability to regenerate their crab exoskeleton as their bodies are still growing, light was thought to be a cycle that never end.
Furthermore, according to mythology, one day Zeus spoke to a crustacean who confessed that walked sideways to cheat time and maintain the same age forever.