Fortunately, I wasn't caught out on this because another American friend told me about it shortly after I moved here (as she had been caught out, embarrassingly) - it's the word - spaz.
Growing up, I wasn't ever aware of the word's origins and as children we would say things like 'Oh I'm (or you're) such a spaz!' to mean hyper or overly energetic, silly/awkward & cutting up, that sort of thing.
In fact, 'spaz' is a shortened/altered form of spastic, referring to people with spastic paralysis - the congenital form of which is known as cerebral palsy. <--- I cribbed this from
here.
I think that the way I used the word as a child is still prevalent in the US (not saying it's right), but the British do (and rightly so I would say) consider it horribly offensive.
This was really brought home to me - the word's hurtfulness, when speaking about it with one of my best friends here - from Canada, who has cerebral palsy. And her point being it is a hurtful term and it would have been used to refer to
her. I felt badly for ever having used it.