Originally, 卍 (manji) is a mirror image of the ancient religious icon called the “swastika symbol”, and has been used in many countries other than Japan (See the Wikipedia article for the details). In Japan, it is used as a map symbol for temples.
* There was a discussion about whether or not Japan should abolish this symbol due to its association with Nazi German. However, the Japanese government decided to leave it unchanged since it has a very long history as a symbol of divinity and spirituality in Buddism.
マジ卍 (maji manji)
Recently (2017~2019), teen girls (especially high school female students) started to use this symbol as a slang term to describe their excitement, especially in the form of the phrase “マジ卍”. It seems (well, I’m not a teen girl so not too sure) it is usually put at the end of a sentence, like “かわいい！マジ卍”, which means “So cute! Awesome!” and “マジ卍” indicagtes one’s excitement.
Although this slang usage of the “word” became popular a couple of years ago, it seems it has become old-fashioned already among real teen girls. Therefore, if you attempt to throw this word in a text/conversation, you may rather end up sounding like a weirdo who pretends to know the Japanese young cultures very well.
See also: Meaning of マジ (maji)
It is used as a map symbol for temples. As a slang term, it’s usually put at the end of a sentence to indicate one’s excitement.
A: It seems the train comes in one minute
B: Wow, that’s so awesome!
やばい “awesome/oh my God/dangerous”