Relating to or describing something that lasts for nine hundred years, from Latin 'nongenti' (nine hundred).
Likely a learned or technical term formed from Latin 'nongenti' (900) with the suffix '-ennial' or similar, though this is an extremely rare term with limited historical usage.
Words like 'hentenian' are so specialized and rare they barely exist in actual use—they're more like theoretical possibilities of English word formation. This shows how many words *could* exist in English but never catch on because we don't need them!
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