From the dictionary:
professional - following an occupation as a means of livelihood or for gain: a professional builder.
The difference between "driver" and "professional driver" is that one of them make's a living from it
In Ukrainian you would say професійний водій about someone whose profession is driving and водій професіонал about a very talented driver
Doesn't it simply mean that you do something as you main occupation and earn money doing so (as opposed to doing it every so often in your spare time)? Of course, it means you are qualified enough that other people are willing to pay you (also, you have lots of practice). Full-time occupation does have a certain standard of quality.
Which is not to say that "professional" does not get used as a loose synonym of "good at", with "very professional" being an even more favourable characteristic and "unprofessional" meaning "incompetent, performing poorly".
I mean, Ukrainian does have професіональний and непрофесіональний with similar connotations.
If you tell me you are a professional hair dresser then I assume you do rich peoples hair, (you don't work at Supercuts). You are correct in that the word professional is an adjective, but it defines the top level of a profession. Driver vs professional driver is to say Fedex vs Nascar, hair dresser vs. professional hairdresser is to say you do peoples hair vs. you do important peoples hair. When you say you are professional at your job, you imply that within that profession, you are at the very top. i.e. a delivery man would never say he is a professional driver. That is how we use the word in spoken english. Hope this helps...
It doesn't have quite the same connotation for me. To me, a hairdresser and a professional hairdresser would be almost exactly the same. I agree that a delivery man probably wouldn't call himself a professional driver, but I wouldn't have a problem with him saying he was a professional delivery man. Perhaps I'm in the minority, but to me "professional" simply implies that you do something as a job.