grade A, n. — Green’s Dictionary of Slang

grade A n.

also grade V

[the US division of milk into three classes: Grade A for infants and children; Grade B for adults only; Grade C for cooking purposes only]

(US) an order of milk in a snack bar.

1936		H.W. Bentley ‘Linguistic Concoctions of the Soda Jerker’ in AS XI:1 45: SHOOT A GRADE ‘A’. Glass of fresh milk.
1966		in Newark (OH) Advocate 21 May 3/3–4: Grade ‘V’ or squeeze one is milk.