'Stan' means 'land'. So, Afghanistan = Land of the Afghans, Baluchistan = Land of the Baluchis and so on. 'Pakistan' is an exception, though the 'stan' part still means 'land'. This name was created by Choudhary Rehmat Ali, a Muslim intellectual, as long ago as 1933. He used the initial letters of the regions that finally went to make up the country at its foundation in the late 1940s. It could, in theory, have been called 'Kapistan', 'Apkistan' etc. As luck would have it, however, Pakistan meant 'Land of the Pure', so that is the combination that he chose.
It does not, in other words, mean 'Land of the P's'! That is, it is not an abbreviation in the same way as 'Brit' from 'Britain/Briton' is. (Obviously, I have not spelt the last word out, as AnswerBank does not permit it, as I understand it.)