A type of extreme love for Scottish rock trio Biffy Clyro.

Symptoms commonly include;

excessive listening to Biffy Clyro records
excessive watching of Biffy Clyro videos
wishing you are Scottish if you're not Scottish
actually being able to keep up with all their tattoos
having yourself tattoed with their lyrics/artwork
knowing the exact shape of Simon Neil (the singer's) ass
knowing the in's and out's of the various hair/beard styles they have sported over the years.