No idea what happened there as I ever meant to type anything in above post. Sorry about that.
It is? I had no idea Tolkien was following Walde's footsteps!The latter part of the word is explained as being the later Mannish (Celticized/Welshified) version of Elvish ‘atáni’.
That is, the ‘t’ lenited, the vowels i-mutated/raised, and the ‘n’ palatalized.