Used for someone deeply passionate about a specific hobby, activity, or subculture. Often appears in compounds like 映画フリーク (movie buff) or ダンスフリーク (dance freak).
He's a fitness freak and goes to the gym every day.
That movie buff apparently watches 300 films a year.
freakshake
Loanword from English 'freak', borrowed into Japanese with the same range of meanings: enthusiast and abnormal creature.