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).
Primitive elvish
nath
root. welcome, be kind to
nathlo
noun. guest
năta
noun. thing
at(ar)
root. father
atar
noun. father
wairē
feminine name. Weaver
A root appearing with Notes on Names (NN) from 1957 glossed “welcome, be kind to”, with what appears to be a primitive word and its Sindarin derivative: ✶nathlo and S. nathal “guest” (PE17/141).