black belt
In martial arts, the belt itself and the rank it represents. Also used metaphorically for a high level of skill.
He holds a black belt in judo.
It takes many years to earn a black belt.
Compound of 黒 (くろ, 'black') and 帯 (おび, 'belt'). The term originates from the ranking system in Japanese martial arts where a black belt signifies a high level of proficiency.