The original meaning of the letter name is uncertain, but it is likely an artificially altered pair with edad, much like Gothic pairþra, qairþra, and may refer to "yew".
The medieval glossators all assign "yew" as the meaning of the letter name referred to by the kennings, though Idad is not a word attested in its own right.
Idad as "yew" is glossed by these later commentators as deriving from a modified form of ibar originally.
McManus suggests that the original letter names for edad and idad were likely *eburas (or *esox) and *iwas, hence their values [e] and [i] respectively, with confusion arising in the medieval period as the language evolved.
[2] In the medieval kennings, called Bríatharogaim or Word Ogham the verses associated with idad are: sinem fedo - "oldest tree" in the Bríatharogam Morann mic Moín caínem sen - "fairest of the ancients" in the Bríatharogam Mac ind Óc lúth lobair (?)