As the basis for “father” words, √AT and its extended form √ATAR date all the way back to Tolkien’s earliest ideas. The root itself did not explicitly appear in the Qenya or Gnomish Lexicons of the 1910s, but forms like ᴱQ. atar, G. †ador “father” indicate its presence (QL/33; GL/17). The root ᴹ√ATA “father” did appear in The Etymologies of the 1930s with derivatives ᴹQ. atar, N. adar (Ety/ATA) and the base √AT(AR) “father” was mentioned again in late 1960s notes on Eldarin Hands, Fingers and Numerals (VT48/19). In this late period, the Elvish words for “father” remained Q. atar and S. adar (PM/324).
Adûnaic
attô
noun. father
A noun for “father” (SD/434). Tolkien gave two forms of this word, attû and attô, with no indication as to which would be preferred. For reasons similar to those given in the entry for ammê “mother”, my guess is that attû is an archaic form, and attô was preferred by the time of Classical Adûnaic. This word is probably related to the Elvish root √AT(AR) “father”, perhaps from Primitive Elvish ᴹ✶atū.