Crab wasn't actually called "Crab." He originally had a proper name—one that marked the Y chromosome, mitochondria, and an individual's name.
However, that name had been discarded by him.
The reason he was called Crab was because of his prosthetic body.
As the Nameless Swordsman, who had saved his life once, explained, in the Old Era there was a creature called "arthropod." Their external jaws and teeth were actually specialized "limbs," evolved from the limbs of ancient ancestors.
And the way he played the guitar with that extra pair of prosthetic hands under his body was reminiscent of a large crab chewing on something.
So Crab stuck with this name from then on.
Before that, he had changed names two or three times. To him, a name was just a milestone on the road of life.
This name still meant quite a lot to him.
