Back-formation from Old Irish fiur, phiur, lenited form of siur, from Proto-Celtic *swesūr, from Proto-Indo-European *swésōr. Old Irish s, when from Primitive Irish *sɸ- (from Proto-Celtic *sw- and *sp-), was lenited as /ɸ/ (spelled ⟨ph⟩ or <f⟩), but Old Irish s, when not from Primitive Irish *sɸ (< *sw-/*sp-), was lenited as /h/, i.e. *sɸ > s, but *hɸ > f.
|Scottish Gaelic mutation|
|Note: Some of these forms may be hypothetical. Not every possible mutated form of every word actually occurs.|