In a way that contains or is characterized by fat; done with a fatty quality.
From 'fatty' (adjective) plus the adverb-forming suffix '-ly', which converts adjectives to describe how actions or descriptions occur.
Adverbs ending in '-ly' can be tricky when the adjective already ends in 'y'—you drop the 'y' and add '-ily', which is why 'fattily' looks strange but follows the pattern of 'happily' and 'angrily'.
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