fiend 
/ˈfi:nd/ noun
plural fiends /ˈfi:nd/ noun [count] 1 a : an evil spirit : a demon or devil
b : a very evil or cruel person
▪ a murderous fiend ▪ a fiend in human form 2 informal a : a person who is very enthusiastic about something : fanatic
▪ He's a real golf fiend. ▪ a fresh air fiend ▪ He's a fiend for physical fitness. [=he is very devoted to physical fitness]
b : a person who is addicted to a drug or to a kind of behavior
▪ a dope/drug fiend [=addict] ▪ a sex fiend
b : a very evil or cruel person
▪ a murderous fiend ▪ a fiend in human form
▪ He's a real golf fiend. ▪ a fresh air fiend ▪ He's a fiend for physical fitness. [=he is very devoted to physical fitness]
b : a person who is addicted to a drug or to a kind of behavior
▪ a dope/drug fiend [=addict] ▪ a sex fiend
like a fiend
chiefly US, informal — used to say that someone does a lot of something ▪ She's been working like a fiend. [=working very hard] ▪ He smokes like a fiend.
