A chubby, plump person
"The chubby toddler waddled happily toward his mother."
sufficiently fat so as to have a pleasing fullness of figure
"a chubby child"
"pleasingly plump"
Of a person: slightly overweight, somewhat fat, and hence plump, rounded, and soft.
"The chubby toddler waddled across the room, his small hands resting on his round cheeks."
In plain English: Chubby means slightly fat but usually in a cute way, often used to describe someone with soft cheeks or arms.
"Her chubby cheeks made her look very cute in the photo."
Usage: Use "chubby" to describe someone with a slight excess of weight that gives them a round or soft appearance, often implying cuteness rather than heaviness. It is more affectionate and specific than the neutral term "overweight," which lacks any connotation of plumpness.
Recorded since 1611, from chub ("short, thick fish species used as bait"; used metaphorically since 1558 for "lazy person"), an assibilated form of cub ("a lump, heap, mass") and cob, from Middle English *cubbe (found only in derivative cubbel ("a block to which an animal is tethered")), from Old Norse kubbr, kumbr ("block, stump, log") and/or Old Norse kumben ("stumpy"), equivalent to chub + -y. Cognate with dialectal Swedish kubbug ("fat, plump, chubby"). More at chub.