pro among pros; best of the best; top tier
Emphatic expression meaning someone who is exceptionally skilled even among professionals; often used in sports, business, or creative fields.
He's a pro among pros, so you can trust him with any difficult job.
That player is called the best of the best, and sure enough, he stays calm under pressure.
Compound of プロ (professional) + 中 (among) + の + プロ, literally 'a professional among professionals'. The exact origin is uncertain, but it is a natural emphatic construction in Japanese.