A muso is a person who is obsessed with music.
Their record collection will contain music and artists nobody else has heard of, and if they believe that an artist is becoming popular they will deny they ever listened to them and quickly dispose of any evidence.
If they come around to your house they will make a beeline for your music collection and then criticise everything in it just to make sure you know they are cool.
As a sideline they will take up the acoustic guitar and then force you to listen to their "revolutionary" take on what music could be like. It will undoubtedly be dreadful and unlistenable.
You're such a muso, if other people have heard of it you hate it.