/ fɜːθ; NAmE fɜːrθ/ noun
(especially in Scottish place names) a narrow strip of the sea that runs a long way into the land, or a part of a river where it flows into the sea :
the Moray Firth
the Firth of Clyde
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WORD ORIGIN
Middle English (originally Scots ), from Old Norse fjo̧rthr . Compare with fjord .