"dope fiend" does not mean a pothead. Weed is only called dope by those who have never done any drugs, and a fiend is someone hopelessly addicted to a substance so much that they would suck a strangers dick for it.
(verb) Prison/street slang. To blindside someone. To attack and hit someone from the blind side, suddenly and without warning. It implies the actions a dope user as being unpredictable and potentially violent.
"He just ran up from behind and dopefiended on the guy."
"New employees say things at staff meetings that make you just want to dopefiend on 'em."