Fr[ae]num

from 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]