Relating to or resembling a cave or cavern; cave-like (archaic or technical term).
From 'auntre' (from Latin 'antrum' via Old French) plus the adjectival suffix '-ous' (from Latin '-osus'), creating an adjective meaning 'having the quality of a cave.'
Latin adjective suffixes like '-ous' made the jump into English vocabulary in wholesale batches, particularly after the Norman Conquest, which is why Latin-derived words often sound more formal and technical than Old English ones.
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