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Webster 1913 Edition

adv.

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

Jer. Taylor.

adv.

Very greatly; to a great extent or degree; as a space vastly extended. Men differ vastly in their opinions and manners.

vastly ‎(comparative more vastly, superlative most vastly)

Greatly, in a vast manner.
- 1913, Robert Barr, chapter 1, in Lord Stranleigh Abroad:
  “I have visited my quarters, and find them very comfortable. […] Steerage is like everything else maritime […] vastly improved since Robert Louis Stevenson took his trip third class to New York.”

greatly, in a vast manner