The Collaborative International Dictionary of English v.0.48:

Frenum \Fre"num\, n.; pl. E. Frenums, L. Frena. [L., a
   bridle.]
   1. (Zool.) A cheek stripe of color.
      [1913 Webster]

   2. (Anat.) Same as Fr[ae]num.
      [1913 Webster]