Superlative form of chubby: most chubby.(adjective)
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Use "chubbiest" in a sentence
"Months of desert heat dropped pounds off even the chubbiest soldier."
"It concluded that the slimmest people ate the most carbs (in the form of whole grains, fruits and vegetables), and the chubbiest people ate the fewest carbs."
"With an adult obesity rate of 33%, Mississippi gobbled its way to the "chubbiest state" crown for the fifth year ..."