Green’s Dictionary of Slang

1. an inexperienced, naive person, a fool.

1623		H. Cockeram Eng. Dict. n.p.: Gnoffe a churle, a foole, a chub.
c.1698		B.E. Dict. Canting Crew n.p.: Chub, c. he is a young chub, or a meer chub, c. very ignorant or inexperienced in gaming, not at all acquainted with Sharping.
1709		N. Ward Rambling Fuddle-Caps 16: Whenever you meet with a generous Chub, You may sneak out a Jest.
1719		A. Smith Lives of Most Notorious Highway-men, etc. (1926) 204: Chub, one easy to be cheated. He is a young chub, or mere chub, i.e., very ignorant, or unexperienced in gaming, not at all acquainted with sharping.
1725		New Canting Dict.
1737, 1759, 1760, 1776		Bailey Universal Etym. Eng. Dict.
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue n.p.: He is a young chub, or a mere chub; i.e. a foolish fellow, easily imposed on: an illusion to a fish of that name, easily taken.
1811		Lex. Balatronicum.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.

2. a rustic simpleton; thus chubbish adj.

1666

‘London Undone’ II: But that those Hobnail’d Clowns should be so chubbish.

3. (UK Und.) a sharper.

c.1698		B.E. Dict. Canting Crew n.p.: The Chubbs, toute the Blosses, they Smash, and make them brush, c. the Sharpers catch their Mistresses at the Tavern, making merry without them, Kick them down Stairs, and force them to rub off.
1725		New Canting Dict. [as cit. c.1698].
1737, 1759, 1760, 1776		Bailey Universal Etym. Eng. Dict. [as cit. c.1698].

4. (also chubster) a fat person; the fat on their body; also as adj.

1838		J. Hogg Tales (1866) 372: When the bishop flung the water on your boy’s face, how the little chub looked at him.
1933		J.E. Grinstead ‘Old Pard’ in Goodstone Pulps (1970) 66/2: He would [...] say to the barkeep: ‘Just leave it there, Chub.’.
1967–70		in DARE.
1992		J. Mowry Way Past Cool 174: The baby chub padding his chest out [...] promised muscle to come.
2012		J. Díaz This Is How You Lose Her 154: Bitch made Iggy Pop look chub.
2018		I. Welsh Dead Man’s Trousers [25]: — Cheer up, hen, shouts a [...] fat cunt in a Hibs strip that clings to him like a body stocking in a chubster’s sex club.

5. (US campus) a child; a baby.

1896

W.C. Gore Student Sl. in Cohen (1997) 5: chub n. [Derivation from cherub suggested.] Child, baby.

6. see chubby n. (3)