"Fy nghas beth"
Translation:My pet hate
It might be easier to think of these as idioms:
Fy nghas beth yw... (My pet hate is...; The thing I hate most is...)
Fy hoff beth yw... (My favourite thing is ...)
cas and hoff are two of a small number of adjectives which can have slightly different meanings when used before or after the noun. Putting cas and hoff after a noun gives us:
y ci cas; y gath gas (the nasty dog; the nasty cat)
ffrind hoff (a dear friend) (not such a common usage, but you do meet it sometimes)