meal vs square meal

meal and square meal both are nouns.

Word	Noun	Adjective	Verb	Adverb
meal	Yes	No	No	No
square meal	Yes	No	No	No

As nouns, square meal is a hyponym of meal; that is, square meal is a word with a more specific, narrower meaning than meal:

meal (noun)	square meal (noun)
the food served and eaten at one time	a substantial and nourishing meal
coarsely ground foodstuff; especially seeds of various cereal grasses or pulse
any of the occasions for eating food that occur by custom or habit at more or less fixed times