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Webster 1913 Edition

adv.

In a monstrous manner; unnaturally; extraordinarily;

as,

monstrously

wicked

“Who with his wife is monstrously in love.”

Dryden.

adv.

In a manner out of the common order of nature; hence, shockingly; terribly; hideously; horribly; as a man monstrously wicked.

To a great degree; enormously; extravagantly.

Who with his wife is monstrously in love.

monstrously ‎(comparative more monstrously, superlative most monstrously)

In a monstrous manner.
- 1859, Charles Dickens, The Haunted House
  She went about the house in a state of real terror, and yet lied monstrously and wilfully, and invented many of the alarms she spread, and made many of the sounds we heard.

in a monstrous manner