In a fat manner; heavily or with abundant flesh; or expressed in a blunt, obvious way.
From the adjective 'fat' plus the adverbial suffix '-ly' (in the manner of), creating an adverb describing how something is done or appears.
The word 'fatly' is rarely used today, but it appears in classic literature to describe both physical appearance and speaking style—calling someone's words 'fatly spoken' meant they were blunt and obvious.
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