Tracing the conceptual development of this root is difficult. Its earliest precursor might have been unglossed ᴱ√FAGA in the Qenya Lexicon of the 1910s with the derivative ᴱQ. fâ = ᴱQ. vilna “air” (QL/37) with Gnomish cognate G. Fâ “lower airs” (GL/33). G. faf- “puff, blow, pant” may also be related (GL/33).
In The Etymologies of the 1930s, however, there was the root ᴹ√PHAU̯ “gape” with derivatives ᴹQ. fauka/N. faug “thirsty” (Ety/PHAU; EtyAC/PHAU). The Noldorin derivative was used in the names N. Anfauglin “Jaws of Thirst” (SM/115) and N. Fauglith “Thirsty Sand; Gasping Dust” (LR/132) from contemporaneous Silmarillion drafts. Sindarin variants of these names S. Anfauglir “Jaws of Thirst” (S/180) and S. Anfauglith “Gasping Dust” (S/150) continued to appear in later versions of The Silmarillion.
In Quenya Notes (QN) from 1957 Tolkien gave a new gloss for the root √PHAW as “emit (foul breath etc.)” serving as the basis for Q. foalóke, unglossed but perhaps “✱breath-dragon” (PE17/181). The root √PHAW reappeared again in 1964 notes on Dalath Dirnen (DD) as the basis for Q. foa and S. faw, all unglossed (PE17/181). Finally ✶phā appeared in some notes from 1968 as an example of a primitive monosyllablic noun, with glosses “puff, (?blow)” or “breath, puff of breath” and probably-related forms fawa and foa (VT47/34-35). All these hints at a possible restoration of the sense of early ᴱ√FAGA from the 1910s: “emit (breath), puff, blow”.
Neo-Eldarin: For purposes of Neo-Eldarin, I think it is best to assume √PHAW had its 1950s-60s meaning “emit (breath), puff, blow”, but that ᴹQ. fauka and N. faug “thirsty” were derived from another root ✱√PHAWAK or ✱√PHAUKA, either an extension or unrelated.
A root having to do with “exhalations (as mists upon water or steams and the like)” serving as the basis for unglossed √PHAY (> Q. fëa “spirit”) and √PHAN (> Q. fanya “cloud”) in notes from 1957 (NM/237). Elsewhere both √PHAN and √PHAY had more divergent meanings, and so may have been disconnected from this root. However, Tolkien mentioned a primitive noun ✶phā “breath, puff of breath” in notes from 1968 with an extended form fawa (VT47/35), so that the root √PHAW “emit (foul breath)” from 1957 notes may have been related (PE17/181).