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Webster 1828 Edition

adv.

In a manner unbecoming or improper.

Incongruously; as a man and wife unsuitably matched.

unsuitably ‎(comparative more unsuitably, superlative most unsuitably)

In an unsuitable manner.
- 1882, Robert Louis Stevenson, "Samuel Pepys" in Familiar Studies of Men & Books,
  Dearly as he loved to talk, he could not enjoy nor shine in a conversation when he thought himself unsuitably dressed.