fiend
n.someone who is very interested in or obsessed with a specific activity. It can also mean a person who is very cruel or wicked.
n.an individual with an intense, obsessive interest in a specific activity or substance; alternatively, a person of great wickedness or cruelty. Often used in compound forms to describe enthusiasts.
“My brother is a total fitness fiend.”
“As a self-proclaimed coffee fiend, she refuses to start her morning without a double shot of espresso.”
“The antagonist was portrayed not as a mere criminal, but as a calculating fiend whose every action was designed to cause maximum psychological distress to his victims.”
Commonly used in informal compounds like 'caffeine fiend' or 'exercise fiend' to denote obsession.
“Hamilton worked out like a fiend for the role, and her resulting lean-yet-ripped physique made her an action heroine on par with Sigourney Weaver’s Ripley.”
“For years, crime-addled Canadians have called for more information on the fiends living among us.”