More containing fat or having more fat content than something else.
From 'fatty' (adjective meaning containing fat, from Old English 'fæt' meaning fat) plus the comparative suffix '-er', which compares two things.
Comparative adjectives let us rank things—fattier is the middle ground between fatty and fattiest, which is why you'll hear nutritionists say 'this cut of meat is fattier than that one' when comparing options.
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