I am overweight because my wife's cooking is delicious but full of fat and sugar. The only way to get a good square meal is to eat out. British war ships in the 1700s including the HMS Victory did not have the best of living conditions. A sailors breakfast and lunch were sparse meals consisting of little more than bread and a beverage. But the third meal of the day included meat and was served on a square tray. Eating a substantial meal onboard a ship required a tray to carry it all. Hence a "square meal" was the most substantial meal served.