a person from Asia, especially Vietnam or elsewhere in Southeast Asia.
I'd also argue nobody knew what "slope" meant until people started to moan that Clarkson said it.
The fallout seemed surreal to me at the time. I felt like Clarkson himself and whoever complained were the only two members of the entire human race who would apply any racist meaning to the word.
Before this particular Top Gear episode, if you'd have asked me to write down a list of all the words that I am aware get used as racist slurs, I would never have even remotely thought of the word "slope". I have only ever known it to mean e.g. "gradient". I expect the same is true of 99.9% of people who saw the original broadcast.
Presumably the word's use in this sense is limited to certain people, e.g. those over a certain age, or those who went to posh school, or only really known outside of the UK, or whatever.
If you want an innuendo, I'll give you one.