A myojine is an attention whore, often called that way on online game forums.
Likes to make things up to make him or herself more intresting while in the mean time making a fool out of him or herself. All the feedback he or she gets is good, it doesn't matter if it is positive or (mostly) negative.
Will put everything about their life (often non-fiction) on online forums in posts to try and make drama, also seen as a troll.

A buzz word being used to refer to creators and influencers forming or encouraging unhealthy relationships with their fans. Usually used in the context of young audiences feeling they have a relationship with the influencer due to the huge amount of the influencer’s content they consume. The influencer in question, however, is unaware of the fan’s existence other than the general knowledge of having fans. Common examples include Dream and his stans. These relationships are exacerbated by the influencers confirming they have more than a creator-to-viewer relationship.

The word is circulating due to various YouTube essays about the phenomenon using the word, and therefore people who enjoy complaining about influencers repeating what they heard in the video in any comments section they can find.