A ravine or narrow wooded valley (dialectal or archaic British term).
From Old English 'gīl' or related to Middle English 'gile,' possibly from Scandinavian roots. The word appears in place names like 'Nightingale' which comes from 'nightingale' (the bird of the gile/ravine).
Words like 'gile' are linguistic fossils—they've almost completely vanished from modern English but survive frozen in place names across Britain. 'Nightingale' actually meant 'the nightingale that sings in the ravine,' showing how old words get fossilized in newer compound words.
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