This term has been taken from the Japanese language. It describes agressive fans who have bad manners on the internet, concerts and threaten lives concerning Johnny's (A Japan entertainment agency which focuses on males).
They even plan an attack on a group of JEs, because they like the other group more. They'll go as far as bringing knives to demonstrate their support.
Rumor: Nino's thumb was once broken because of a yarakashi.
Rumor: Yarakashi's love to insult aiba during concerts because they think he is 'stupid'.