Origin of chubbyfrom chub
A chubby cat.
When you have a toddler who is round and plump, this is an example of someone who would be described as chubby.
Origin of chubbyProbably from chub ( from the plumpness of the fish )
(comparative chubbier, superlative chubbiest)
recorded since 1611, from chub (short, thick fish species, used as bait), used metaphorically for "lazy person" since 1558; perhaps influenced by Old Norse kumba "log" and/or kumben "stumpy."