He's a 'fiend' for physical fitness. [=he is very devoted to physical fitness]
She's been working 'like a fiend'. [=working very hard]
a dope/drug 'fiend' [='addict']
(..) Or I'm going to find out what this weird looking gun does. No don't! Don't shoot that gun. I'll just go get him. Unhand me you fiend! It, it's strength's to much. Oh, I work out. Argh, it's really paying off. You're so fit, and strangely charismatic. Are you okay? (..)2010 Megamind