piuthar

Etymology
Back-formation from, , lenited form of , from , from. Old Irish s, when from *sɸ- (from Proto-Celtic *sw- and *sp-), was lenited as /ɸ/ (spelled ⟨ph⟩ or ⟨f⟩), but Old Irish s, when not from  *sɸ (< *sw-/*sp-), was lenited as, i.e. *sɸ > s, but *hɸ > f.

Noun

 * 1) sister, female sibling