How is 'very, thanks' wrong? Admittedly it requires some sort of specific question beforehand, but it's not that unlikely. 'Dych chi'n hapus nawr? Iawn, diolch.' Right?
iawn acts as an intensifier, 'very', when it follows the adjective or adverb that it is intensifying:
- da, da iawn - good, very good
- cyflym, cyflym iawn - quick, very quick
- Mae e'n ymddwyn yn ddrwg iawn - He is behaving very badly.
On its own, as in the phrase Iawn, diolch in response you a greeting asking how you are, it means 'Fine'.