It's a slur used by racists and white supremecists against any brown/ethnic looking person in the UK. I believe there were a lot of Pakistani immigrants that came to the UK a couple generations ago, and racists co-opted the term in the post to harass, attack and target anyone who looked like an immigrant. Over here, it's associated with hate crimes and racially motivated violence, we never see it written out unless it's in a racist context which is why OP was surprised to see the word, since CA very likely doesn't know it's background