In the eighteenth century […] ‘an ordinary’ was a meal that was equivalent to the French term plat de jour (the dish of the day). So, in Henry Mackenzie’s The Man of Feeling from 1771, two well-to-do men are walking by a park when ‘they observed a board hung out of a window, signifying, “An excellent ORDINARY on Saturdays and Sundays.” It happened to be Saturday and the table was covered for the purpose’ […]. There is no sense that the two men are going for the cheap or the measly option here. An ordinary in this sense was the meal on which most care and attention was lavished, that used the freshest produce and the best cuts of meat. It was also what you might eat as a regular customer of the café or restaurant. An “ordinary” suggests both the care and effort of the cook or chef and a community of diners who know how to choose the best option because they respect the decisions and skills of their patron.
Ben Highmore. Ordinary Lives: Studies in the Everyday. Routledge, 2011.