Spa is an abbreviation of the word spastic which is a medical condition related to muscle spasm or cerebral palsy, however it is important to note that the word spa is not to be taken as offensive to sufferers of these conditions.
It is more commonly used as a light hearted insult regarding idiotic behaviour.
An American equivalent might be asshole while the British counterpart might be plonker.
--Uh, Becky, are you OK?
--Sorry, I suffer from Senseless Panic Attack Syndrome... I'm a Spas.