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Webster 1913 Edition

Noun.

[L.

dementia

, fr.

demens

mad. See

Dementia; loss of mental powers. See

Noun.

Madness.

demency ‎(uncountable)

(now rare) Madness; dementia.
- 1969, Vladimir Nabokov, Ada or Ardor, Penguin 2011, p. 263:
  no direct access could be obtained to the banned, or burned, books of the three cosmologists, Xertigny, Yates and Zotov (pen names), who had recklessly started the whole business half a century earlier, causing, and endorsing, panic, demency and execrable romanchiks.