My brother in law, who was born and bred in the toon and grew up there in the 50's, used the term and always said is was about welding skills and that the Wearside shipyard welders could only mackem and the ships had to be taken to the Tyne to finish them off [tackem]. I was never sure, in fact I often thought it was the poor sad people from Sunderland who invented the word because Newcastle people had their own nickname and they didn't. So a derivation from the second half of the 20th century does not surprise me.
Of course I must point out that not all Wearsiders are from Sunderland. There are plenty of intelligent people from Chester le Street and Durham who support a proper team in black and white stripes.
How did I get into this on Mudcat?