Adad in Arabic means number, count, sum, as in the Quran:
It is He Who created the sun radiating and the moon shining and appointed positions for it, for you to know the number of the years, and the account; Allah has not created it except with the truth; He explains the verses in detail for the people of knowledge. (Quran 10:5)
It is not used as a name by Arabs. It is pronounced as (U)n(DE)rstan(D).