groove in a shamisen player's fingernail
A physical groove worn into the fingernail from repeated friction against the shamisen string. Specific to shamisen performance.
shamisen playing ability
Metaphorical extension: the groove in the nail comes to represent the player's skill or experience. Used in contexts discussing a performer's proficiency.
Playing the shamisen for many years creates a groove in the fingernail.
His shamisen skill is still shallow, but he practices diligently.
Compound of 糸 (ito, 'string') and 道 (michi, 'path, way'). Literally 'string path', referring to the groove worn into the nail by the string, and by extension the player's skill.