Very old or ancient; belonging to the distant past (used in the phrase 'olden days' or 'olden times').
From 'old' (Old English 'eald') plus the archaic suffix '-en' (used to form adjectives). This suffix survives in words like 'golden' and 'wooden.'
The phrase 'olden days' is actually grammatically weird—most English speakers today wouldn't naturally add '-en' to adjectives, which is why 'olden' sounds poetic and old-fashioned on purpose.
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