Surreptitious, coded football (soccer) slang from North East England. Can either mean 'Fuck the Mags (or Magpies)!' - this being the traditional and original definition as used by the supporters of Sunderland Association Football Club - or alternatively 'Fuck the Mackems!' as bastardised by the supporters of Newcastle United Football Club.
'Magpies' is a geordie
colloquialism for NUFC who play in black and white shirts. 'Mackems' was formerly a derogatory term for the folk from Sunderland as coined by the people of Newcastle, who, jealous of Sunderland's prowess in the noble art of shipbuilding, would say, 'They mak' em and we tak' em!' (ie, the ships). Today, however, the citizens of Sunderland are proud to be associated with the word Mackem and the connotations of its heritage.
Unless you live in the North East of England, you cannot imagine the fierce and intense rivalry between the two cities of Sunderland and Newcastle despite their being separated by only a few short miles. This burning hatred of each other has endured for many centuries and will no doubt continue for many more.
See also Mackem and Magpie