This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

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ad. For the most part; chiefly.

ad. Very strongly; mightily; to a great degree.

ad. Principally; chiefly.

By main strength; strongly; forcibly; firmly.

Greatly; to a great degree; mightily.

Chiefly: principally: as, he is mainly occupied with domestic concerns.

ad. for the most part

"He wants a rematch to reclaim his title mainly for the big payday it would bring—and he wants this, by every evidence, only in order to pay for his daughters' schooling, to keep the family cars from being repossessed, for his wife's peace of mind while she pursues a medical career."

mightily    principally    chiefly   

humanely    plainly    ungainly    vainly