I haven't the faintest idea what this sentence means. It was taken from the book Learning Irish. It goes as follows:

Caith fút ar an gcathaoir sin!

I guess it means something like 'spend on that chair you!' But what is the point of 'fút' in that sentence? Ó Siadhail never showed us any idiomatic constructions of the sort using this prepositional pronoun.

So could anyone please enlighten me on the meaning of this sentence?

Go raibh maith agaibh!

It just means "Sit down on that chair!"

Faoi/fút has the meaning of "down". Baint fút in áit and cur fút in áit both mean "to settle down in a place".

Caith can also mean "throw", or "wear", and a host of other meanings.

You could think of it like this: Caith fút ar an gcathaoir sin! = "Throw yourself down on that chair!" or "Plonk yourself down on that chair!"

a Bhreandán, go raibh míle maith agat!

What you say seems to make perfect sens. I'll take your word for it.

It would have been good though had Ó Siadhail given me first all an example of such a sentence in the grammar. Oh well... those are the pleasures and displeasures of learning Gaeilge.

Slán!