Fòram na Gàidhlig

Does anyone know the etymology of the "Forth" in Firth of Forth? I know how the "firth" part comes in, but I'm having trouble tracking down the etymology of "forth". The Gaelic Wikipedia page for the Firth of Forth lists its Gaelic names as Abhainn Dubh and Uisge Foir(the). It seems like the latter is just a borrowing from Scots or English, suggesting to me that "forth" does not have a Gaelic etymology. Any insights?

Tapadh leibh!

It has several other Gaelic names also. It's not at all clear what it might mean in English - and its current form (Forth) renders a Welsh/Brythonic provenance unlikely. If I mind right, Ptolemy listed it as "Bodotria" or some such. To the Votadini/Gododdin it seems to have been kent as "Iudeu" At one time it was fashionable to try and link the name with "Fortrenn" or Fortriu", one of the ancient Pictish areas once thought to lie in lower Perthshire/Stirlingshire but now known to have been more in the Inverness/Moray area, being cognate with the people the Romans knew as "Verturiones" and later Angles as "Waerteras".

However, Fife itself was traditionally divided into two sections, the NE generally being "Fife" and the SW "Fothriff" (there are variant spellings of that). It doesn't take much metathesis to turn the latter into something rather similar to "Forth", but the thinking is that "Fothriff" actually represented something like "Fo-Fhìobha" or "Lower Fife". It might or might not be related to "Forth".

At one time it was known as the Frisian Ocean - probably as there was well-established trade between various of its ports and Holland (e.g. Kincardine especially, but by no means only).

And there's also the fact you've got the Solway Firth, the Firth of Clyde, the Moray Firth and the fourth one...

Many of the strange terms mentioned above are thought to contain elements referring to the sluggish nature of the sea in this particular estuary - and that also applies to, e.g. "Foirthe" in Gaelic. One thing that is almost certainly the case, however, is that the River Forth is a "back-formation", i.e. the firth gave its name to the river rather than vice versa. For the River Forth isn't exactly Scotland;s answer to the Amazon, is it? I suspect it puts less water in the the firth than the Carron does, for instance.

For a fuller treatment, I recommend Taylor & Márkus, "The Place-Names of Fife" Vol, 1.