One of the mysteries of the English language finally explained.
(The celebration of) a nine hundredth anniversary.
1960s; earliest use found in The New Statesman. From post-classical Latin nongenarius containing nine hundred (5th–6th cent.; from nongeni nine hundred each (5th–6th cent.; shortened from nongenteni from classical Latin nōngentī nine hundred + -ēnī, suffix forming distributive adjectives: see nonagenarian) + -arius), after centenary.
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