The Collaborative International Dictionary of English v.0.48:

Fraenum \Fr[ae]"num\, or Frenum \Fre"num\, n.; pl. E.
   Fr[ae]nums, L. Fr[ae]na. [L., a bridle.] (Anat.)
   A connecting fold of membrane serving to support or restrain
   any part; as, the fr[ae]num of the tongue.
   [1913 Webster]