In Japanese, maji-manji マジ卍 means the same thing as maji-yabai マジやばい. It's a JK slang used when something is intense, either intensely good, cool, awesome, epic, or intensely bad, horrible, awful, terrifying, dangerous.
The manji 卍 is also used as an internet slang when you're hyped up, excited for something.
See the article about the usage of the swastika in Japanese for details. This article is just for examples of usage.