The forfeda (sing. forfid) are the "additional" letters of the Ogham alphabet.

Uilleann means 'Elbow'.

Later Medieval scholars believed that all of the letter names of the ogham alphabet were those of trees, and attempted to explain the bríatharogaim in that light. However, modern scholarship has shown that only eight at most of the original twenty letter names are those of trees, and that the word-oghams or kennings themselves support this.