One who gets unusually fervent about a hobby or vocation; to the point that it dominates conversation, free time and daily life.
Who's that driving around in that pickup truck is that with all the red lights and chrome sirens? It has a sticker of Calvin peein' on a fire in the back window and a "My other car is a fire truck" license plate frame.
Oh, that's Freds. His is a stemwacker volunteer fireman.