The earliest appearance of this root was ᴹ√TOTO- “repeat” from Demonstrative, Relative, and Correlative Stems (DRC) from 1948 (PE23/109). The root appeared as √TŌ/OTO in a discussion of prefixes for “back” from around 1959, where Tolkien specified its meaning as “back as an answer, or return by another agent to an action affecting him, as in answering, replying, avenging, requiting, repaying, rewarding”; Tolkien also considered the forms √UTU/TŪ (PE17/166). In this 1959 note Tolkien crossed √TŌ/OTO through and seems to have replaced it with √KHAN. Tolkien mentioned the root √OT in a discussion of numbers from the late 1960s, but only to specify that “there was no primitive base OT-” (VT47/16).
Primitive elvish
atta
root. ATTA
atta
cardinal. two
atata
root. two, two; [ᴹ√] again, back
atar
noun. father
(a)tata
cardinal. two
at(ar)
root. father
inte
pronoun. they (emphatic)
itte
pronoun. they (emphatic)
khan
root. back
tata
masculine name. Two
tō/oto
root. back
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).