A dialectal or Scottish term referring to the ooze or mud, or a flush of water; possibly a variant of crush or related to marshy conditions.
Scottish English or Northern English dialect, possibly onomatopoetic or related to Old Norse roots for marsh/bog, though etymology is uncertain and the word appears mainly in regional texts.
Scots dialect is full of words for mud, bog, and water in different states—grush might be one of many regional terms that never standardized, showing how much weather-specific vocabulary can vary by region.
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