For a little more context it's the equatable to calling someone a ******.
From Wikipedia:
Quote:
The medical term "spastic" came into use to describe cerebral palsy.[3] The Scottish Council for the Care of Spastics was founded in 1946, and the Spastics Society, an English charity for people with cerebral palsy, was founded in 1951.
However, the word began to be used as an insult and became a term of abuse used to imply stupidity or physical ineptness: one who is uncoordinated or incompetent, or a fool.[4] It was often colloquially abbreviated to shorter forms such as "spaz".
I think it's all about context. If you call someone a spastic or a ****** then it's out of line, but if you're describing something that moves/acts like it has spasms or if you're describing something that's slowing down then spastic and ****** would be ok.