Square meal is an Americanism. It comes from the adjectival use of square to mean sturdy or substantial. There are older, related senses of the adjective square. In the 17th century, for example, square was used to describe someone who could eat and drink copious amounts. From Randle Cotgrave’s 1611 Dictionarie of the French and English Tongues:
Vn ferial beuveur, a square drinker, a faithfull drunkard; one that will take his liquor soundly.
And from Beaumont and Fletcher’s Bonduca from c.1616:
By —— square eaters, More meat I say:...how terribly They charge upon their victuals.
The term square meal itself appears in the late 1860s, but does not become common until the 1880s. From the magazine All Year Round of 19 September 1868:
Roadside hotel-keepers...calling the miners’ attention to their “square meals:” by which is meant full meals.
There are various stories relating this phrase to the types of food (usually four in number) consumed. These stories are not true.
The style of eating, dubbed square meal and once required of plebes, first year cadets, at West Point, where eating utensils must be moved at right angles is derivative of the common usage, not the origin.
(Source: Oxford English Dictionary, 2nd Edition)
Copyright 1997-2015, by David Wilton