'Mac' is a standard nickname in the UK for anyone whose surname begins Mc or Mac. 'Macca' was the nickname bestowed on him by his friends, including the other Beatles, and has been the standard term of affection for him in his own country pretty much since the Beatles became famous.

And Paul McCartney is a native of Liverpool, where they have their own words for all sorts of things, even fellow Brits are sometimes at a loss to understand Liverpudlians.