Derived by Geiriadur Prifysgol Cymru from rhy- (“before”) + geni (“birth”), possibly continuing a Proto-Celtic stem *ɸro-gany-. Compare Ancient Greek πρόγονος (prógonos, “ancestor”) for an equivalent formation.
In the 1990s, Lambert and Schrijver found a possible Gaulish cognate, relating rhieni with Gaulish regenia. They disagreed on the words' shared Proto-Celtic reconstruction, however. Lambert opts for *ɸrogenyā while Schrijver opts for *ɸrogenesa. Both reconstructions face phonological difficulties. Lambert has no explanation for the -i ending in Welsh (final *-yā after a consonant is normally lost in Welsh) while Schrijver assumes a tenuous deletion of weakened -s- in Gaulish (which has many counterexamples).
rhieni m (collective, singulative rhiant)
| Welsh mutation | |||
|---|---|---|---|
| radical | soft | nasal | aspirate |
| rhieni | rieni | unchanged | unchanged |
| Note: Some of these forms may be hypothetical. Not every possible mutated form of every word actually occurs. | |||
R. J. Thomas, G. A. Bevan, P. J. Donovan, A. Hawke et al., editors (1950–present), “rhieni”, in Geiriadur Prifysgol Cymru Online (in Welsh), University of Wales Centre for Advanced Welsh & Celtic Studies