"Most cornfed" is a phrase that often describes someone who has a strong, sturdy build - likely as a result of being raised on a corn-rich diet. Antonyms for the phrase might include "underfed," "emaciated," or "gaunt," all of which suggest a lack of proper nutrition or hardship. Alternatively, one might choose to use antonyms that suggest the opposite of a rural, agricultural upbringing, such as "cosmopolitan," "urban," or "worldly." Ultimately, the antonyms chosen for "most cornfed" will depend on the context in which it is being used and the desired tone of the statement.