A Quenya noun for “thing” derived from the root √NĀ “be, exist” (VT49/30, Ety/NĀ²), so perhaps prehistorically simply “a thing that exists”. Its plural form nati is indirectly attested in the plural únati of its (strong) negation únat “a thing impossible to be or to be done” (VT39/26).
Conceptual Development: This word is well established in Tolkien’s writings, appearing all the way back in the Qenya Lexicon from the 1910s (QL/64). In its earliest iteration, its stem form was natt- and its plural was natsi, where [[eq|[ti] became [tsi]]] as was the usual pattern in Early Qenya. The word reappear in texts and notes from the 1920s (PE14/43, 72; PE15/32, 68, 78). In one early dictionary entry glossed more generally as “affair, matter, thing”, but this entry was deleted (PE15/68); in other early writings the word for “affair” was given as ᴱQ. natto (QL/64). The word reappeared in The Etymologies from the 1930s with a simplified stem form nat- given its Noldorin equivalent N. nad (Ety/NĀ²). The word appeared again in the late 1960s in notes associated with Q. ná- “to be”, where it was given the primitive form ✶năta (VT49/30).
nat noun "thing" (NĀ2); compare únat. VT49:30 lists "năta, nat", but it is unclear whether năta is here a Quenya word or an etymological form underlying Quenya nat.