The Collaborative International Dictionary of English v.0.48:

Demi- \Dem"i-\ [F. demi-, fr. L. dimidius half; di- = dis- +
   medius middle. See Medium, and cf. Demy, Dimidiate.]
   A prefix, signifying half.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Demi \De*mi"\, n.
   See Demy, n.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Demy \De*my"\, n.; pl. Demies. [See Demi-.]
   1. A printing and a writing paper of particular sizes. See
      under Paper.
      [1913 Webster]

   2. A half fellow at Magdalen College, Oxford. [Written also
      demi.]
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            He was elected into Magdalen College as a demy; a
            term by which that society denominates those
            elsewhere called "scholars," young men who partake
            of the founder's benefaction, and succeed in their
            order to vacant fellowships.          --Johnson.
      [1913 Webster]