The Collaborative International Dictionary of English v.0.48:

Dement \De*ment"\, a. [L. demens, - mentis.]
   Demented; dementate. [R.] --J. H. Newman.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Dement \De*ment"\, v. t. [L. dementare, fr. demens, -mentis, out
   of one's mind, mad; de + mens mind. See Mental, and cf.
   Dementate.]
   To deprive of reason; to make mad. [R.] --Bale.
   [1913 Webster]