A firth is a narrow inlet of the sea, in Scotland. The Firth of Forth, across the firth, a wide firth.
It is the Scottish word and it fills the map — Forth, Clyde, Tay, Moray. A firth is usually the estuary of a river and wider than a fjord. The word and fjord are the same Norse word arriving by different routes, which is why they mean nearly the same thing.
1nounfirthA narrow inlet of the sea, in Scotland.
“The Forth Bridge crosses the Firth of Forth near Edinburgh.”
firth is an inlet.
It shares its root with fjord: a long narrow inlet where the sea cuts into the land. In Scotland the old word wore down to firth.
hooks ontofjord
Scots firth comes from Old Norse fjordr, an inlet. Fjord is the same word borrowed later and directly.
Proto-Indo-European
*per-
Proto-Indo-European
*pértus — crossing
Proto-Germanic
*ferþu
Old Norse
fjǫrðr — firth, fjord
Old English
ford
enm-nor
fyrth
Scots
firth
English, today
firth
Built from
Borrowed from Scots firth, furth, from enm-nor fyrth, from either or both of Old English ford and Old Norse fjǫrðr (“firth, fjord”), from Proto-Germanic ferþu, ferþuz (“inlet, fjord”), from Proto-Indo-European pértus (“crossing”), from per- (“to carry forth”) + -tus. The English word is a a doublet of fjord and ford and port and fjard.
Etymology from Wiktionary, CC BY-SA