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

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ad. Greatly, in a vast manner.

ad. To a vast extent or degree; very greatly; immensely.

Like a waste; desolately.

Very greatly; to a vast extent or degree: also in exaggerated colloquial use (see vast, a., 4).

ad. to an exceedingly great extent or degree

"It was an expression vastly different from the tired helplessness she had seen in the faces of men coming home from the war after the surrender."

immensely    extremely    exceedingly    supremely    tremendously    highly   

Astley    ghastly    lastly