I'm guessing it was from childhood as it seems to be a pretty common nickname. From Wiki:
Macca (or "Macker" in Ireland) is a common British Isles nickname for somebody whose surname begins with the Gaelic prefix Mac or Mc (meaning "son of"). It is similar in form to Gazza, Hezza and similar nicknames that arose during the 1990s and early 21st century.
People frequently referred to as Macca include:
* Sir Paul McCartney, musician
* Amy Macdonald, singer
* Steve McClaren, ex-England football manager
* Andrew McLeod, Australian rules footballer and two-time Norm Smith Medallist
* Steve McManaman, footballer sometimes also called 'El Macca'
* Stephen McPhail, footballer
* Gary McAllister, former footballer and manager
* Macbeth of Scotland, former Scottish king and Shakespearean tragic hero
* Gary McSheffery, footballer
* Chris McCormack, Australian triathlete
* Michael "Macca" MacKenzie, a fictional recurring character on the Australian soap opera Home and Away
* Steve McMahon, former footballer
* Bruce McAvaney, Australian Sports Media Broadcaster
* Neil McKenzie, South African Cricketer
* Paul McNamee, Former Australian Tennis Player, now Sports Administrator