An expert at alcohol. Like a specialist~ a professional. Someone who is a specialist at alcohol. Unlike the term "alcoholic", which has a negative connotation, an "alcoholist" is someone who is a specialist at alcohol. They are unlikely to be a drunk, but more akin to a connoisseur of alcohol. This is a compliment, and a term of respect and camaraderie.
That person is a pianist.
That person is a specialist.
That person is a biologist.
That person is an economist.
**That person is an alcoholist.**