Paki, an ethnic slur? I would've thought it was short for Pakistani.
http://en.wiktionary.org/wiki/Paki
Starting with the very first sentence under "Usage notes".
In summary, a strict prefix of some place name or nationality is not automatically a harmless contraction. For instance, "Jap" is not a word which has exactly the same nuance as "person of Japanese descent".
When the Pakistan cricket team tours down here in Australia, they're colloquially referred to as "the Pakis", but that's because we colloquialise EVERYTHING, and has nothing to do with slurring anybody.
He doesn't mention the use of the word Paki by neo-nazi far-right racists.
Edit: in the UK it's almost always a slur. I can see how in other places it's just an abbreviation.