A young salmon that has returned to fresh water for the first time after spending one winter in the sea.
From Scottish/Northern English dialect, possibly from Old Norse 'grilsi.' The term is specific to salmon fishery terminology and has been used since at least the 16th century in Britain.
A grilse is a specific life stage of salmon that's only about two years old and much smaller than adult salmon—knowing this word would make you sound like you actually fish for salmon in Scotland or Norway!
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