This looks like it's related to 'fo:s' (: = fada) and Scottish Gaelic 'fhathast' 'still, yet'
I have a feeling that all three words come from
fofecht-sa/fodecht-sa 'under this time' (fecht = time)
--- On Wed, 6/25/08, Dennis King <[log in to unmask]> wrote:
> From: Dennis King <[log in to unmask]>
> Subject: [OLD-IRISH-L] etymology of "fosta"?
> To: [log in to unmask]
> Date: Wednesday, June 25, 2008, 1:12 PM
> Does anyone know the etymology of Modern Irish
> "fosta" (= also), used
> in Donegal?