The story
From regional form of make + 'em, generally linked to the history of shipbuilding in Sunderland. In relation to the nearby port of Newcastle, Sunderlanders would supposedly say "we mack 'em and they tack 'em", with tack being either a regional form of "take" or referring to any of the nautical senses of tack.
Etymology from Wiktionary, CC BY-SA 4.0.