What is the meaning of "corn ball at work"?

[Oinas]

English Language Tests, Intermediate level

ESL/EFL Test #451 "English Slang Idioms (190)", question 6

"Lester is such a ......... ball at work. The other day, he came to work in a prisoner's costume," Vicky told Michelle.

(a) puff
(b) sleaze
(c) beach
(d) corn

English Language Tests, Intermediate level

ESL/EFL Test #451 "English Slang Idioms (190)", answer 6

"Lester is such a corn ball at work. The other day, he came to work in a prisoner's costume," Vicky told Michelle.

Correct answer: (d) corn

Your answer was: incorrect
"Lester is such a beach ball at work. The other day, he came to work in a prisoner's costume," Vicky told Michelle.
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What is the meaning of "corn ball at work"? Best regards,
Oinas.

[Beeesneees]

Cornball = a person with trite or sentimental ideas. It's an American expression and may not be recognised in the UK.
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Cheers m' dears!