a famous metal railway bridge built in 1889 over the Firth of Forth. People sometimes say that a job is "like painting the Forth Bridge" when they mean that it seems to never end, because the metal bridge takes a long time to paint, and when the job is finished it has to start again.
2a road bridge built in 1964 over the Firth of Forth. It is one of the longest suspension bridges in the world.
Definition from the Longman Dictionary of Contemporary English Advanced Learner's Dictionary.
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