Overview
“Fiend” is slang for someone who uses drugs compulsively, so it amounts to a more derogatory version of calling someone an “addict.” Someone that’s “fiending” is expressing strong cravings to either relapse or continue a bender.
History and Etymology
“Fiend” from Old English originally meant “enemy” or Satan as the enemy of mankind. Around the late 19th century, American media was using “fiend” to imply someone with an excess or uncontrollable urges, such as “autograph fiends” referencing people who hounded celebrities for autographs. Eventually, the term “dope fiend” arose in reference to those who habitually and compulsively sought out liquid opium from dens.