No, it is NOT slang. It is a UK official abbreviation for "Statutory Off Road Notification".
Anyone who owns a car must renew their road tax disc when the current one expires each year UNLESS the vehicle is destroyed OR taken off the public road.
SORN means an official declaration by the car owner that the vehicle is no longer being used on public roads (eg for long term repair/renovation, or because the owner is going overseas for several months).
This means they do not have to pay the road tax and so that central records are kept up to date for the police to check.