But more modernly is used to describe something 'trendy'.
Can be abbreviated to simply 'Rrr' or alternatively, when drunk, can be phrased as 'RRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRR!!!' or 'RRRRREEEEEEEUUU!!!'
More technical terms of the phrase can be 'Reu Dy Fam' (Yours Mother's Reu) to 'Reu Dy Nain ar Dost' (Your Grandmother's Reu on Toast).
Rules for saying 'Reu' are:
1. Must not be said by anyone under the age of 18. Ever. Although the Welsh Assembely is considering lowering the age to 16.
2. Must not be said anywhere outside the Arfon area.
3. Use of the word must not exceed more than 3 times a day. This is number is unlimited to individuals from Dyffryn Nantlle.
4. Can only be put into a sentence that is in Welsh.
There is no other translation anywhere in the world for this word.
Dafydd: "Iawn Kev?" (Alright Kev?)
Johnny Bach: "Reu Dy Hosan, Cont." (Reu of your sock, Cunt.)
Wmffra: "Rrr, 'gia!" (Rrr, boys!)
James: "Ti 'di weld car newydd fi, met?" (Have you seen my new car, mate?)
Osian: "Mae hwnna'n reu, lad!" (That's reu, lad!)