pan fydd yr Wyddfa’n gaws

Literally, “when Snowdon is made of cheese”.

pan fydd yr Wyddfa'n gaws

1. (idiomatic) when pigs fly, never

• bwyta uwd â mynawyd (“to herd cats”, literally “to eat porridge with an awl”)
• tan ddydd Sul y pys (“when pigs fly”, literally “until the peas' Sunday”)
• pan fydd Nadolig yn yr haf a gwsberis yn y gaeaf (literally “when Christmas will be in the summer and goosberries in winter”)
• pan fydd uffern yn rhewi drosodd (“when Hell freezes over”)
• unwaith yn y pedwar amser (“once in a blue moon”, literally “once in the four seasons”)
• unwaith yn y pedwar gwynt (“once in a blue moon”, literally “once in the four winds”)

• R. J. Thomas, G. A. Bevan, P. J. Donovan, A. Hawke et al., editors (1950–present), “pan fydd yr Wyddfa’n gaws”, in Geiriadur Prifysgol Cymru Online (in Welsh), University of Wales Centre for Advanced Welsh & Celtic Studies