"Forever dope" originated from the word "dope". It modifies the level of how "dope" something is to a higher level, better than saying "that is REALLY dope".
1. Someone or something that one enjoys or thinks is great at a high or the highest degree.
2. A topic or an opinion that is agreeable to oneself.
1. "Man, Robert Downey Jr. was so good in Sherlock Holmes! I knew he was cool, but he is forever dope!"
2. "I don't care what you think about Lil Wayne's Rebirth album, because I think it's forever dope".