Somewhat fat or inclined to be fat; mildly overweight or containing some fat.
From 'fat' (adjective) plus the diminishing suffix '-ish', which softens or reduces the intensity of a quality.
The '-ish' suffix is wonderfully vague—'fattish' means you're not quite calling something fat outright, which is why people use words like 'tallish', 'bluish', or 'warmish' when they want to hedge their descriptions.
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