"My pet hate"
Translation:Fy nghas beth
Beth...? - What...? (from the original pa beth? - what? what thing?
Peth - A thing
Cas beth - A pet hate (peth is mutated here because it is a noun following an adjective, cas)
As an adjective, cas does not mean 'hate'. In its usual position after a noun it means 'nasty'.
In the fixed expression cas beth ('a pet hate'), though, where it is before the noun, it just forms a part of the phrase - trying to break the phrase down doesn't really work.