Scottish word used for various coastal inlets and straits
Firth is a word in the English and Scots languages used to denote various coastal waters in the United Kingdom, predominantly within Scotland. In the Northern Isles, it more often refers to a smaller inlet. It is linguistically cognate to Scandinavian fjord and fjard (all from Proto-Germanic *ferþuz), with the original meaning of "sailable waterway". The word has a more constrained sense in English. Bodies of water named "firths" tend to be more common on the Scottish east coast, or in the southwest of the country, although the Firth of Clyde is an exception to this.
https://en.wikipedia.org/wiki/Firth