A word appearing as fā in 1968 notes on primitive monosyllables as a derivative of ✶phā “breath, puff of breath” (VT47/35). Tolkien said this word survived in Quenya, but in a later sentence he wrote Q. fawa, foa, perhaps indicating its more common form was foa. The form foa appeared unglossed in 1964 notes on Dalath Dirnen (DD), and also in 1957 Quenya Notes (QN) as a derivative of √PHAW “emit (foul breath etc.)” and an element in foalóke, probably = “✱breath-dragon” (PE17/181). As such, I consider foa the most likely Quenya form and “breath, puff of breath” its most likely meaning.
Conceptual Development: In the Qenya Lexicon of the 1910s, this word appeared as ᴱQ. foa “hoard, treasure” under the early root ᴱ√FOƷO “hide, hoard, store up, lay up in secret”, also as an element in ᴱQ. Foalôke as the name of a dragon (QL/38).
An (archaic) Parmaquesta (PQ) noun in the Outline of Phonology (OP2) from the 1950s derived from primitive ✶phāy as an example of how ancient final y vanished after long vowels in monosyllables (PE19/104). A few pages earlier, primitive ✶phāy was glossed “flame, ray of light” (PE17/102), so perhaps this was the intended meaning of fá also. But Tolkien said it was given up in favor of the longer form fëa (PE19/104), which elsewhere was usually translated as “spirit”.
Neo-Quenya: Given that this word is archaic and is of uncertain meaning, I would avoid its use for purposes of Neo-Quenya, using words like ᴹQ. alka for “ray of light” and Q. fëa for “spirit”.