Comparative form of 'fanny,' though this form is rarely used in standard English.
From 'fanny' (a colloquial term with varying meanings by region) + comparative suffix '-er'. This is a grammatically possible but uncommon word formation in English.
While 'fannier' is technically valid grammar, it's almost never used—it's one of those words that could exist but doesn't because speakers just don't say it, showing that not every grammatically possible word actually survives in real language.
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