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Webster 1913 Edition

adv.

In a brainsick manner.

adv.

Weakly; with a disordered understanding.

brainsickly ‎(comparative more brainsickly, superlative most brainsickly)

(obsolete) In a brainsick manner.
- Macbeth
  Lady Macbeth: You do unbend your noble strength, to think
  So brainsickly of things.