A tidal wave or bore, especially a high wave caused by tidal flow in certain river estuaries.
From Old Norse 'egor' or similar Scandinavian sources, brought to English through contact with Norse speakers. The term survives primarily in British English for tidal phenomena.
The word 'eagre' (also 'bore') came from Norse seafarers and still describes the dramatic tidal waves in rivers like the Severn—it's a word tied to specific geography that only people living near those rivers use regularly.
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