Garrity means mad and my grandfather (born 1904 in East London) used it all the time to describe men or horses - "he was garrity" or "he went garrity."
I use it to describe going mad, I think that it came from Freddie Garrity as he acted quite mad
It comes from South London. It's because of Garratt Lane. If you were going Garratty Lane, you were going insane.