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A boy was asked to go to the market to buy the necessary food items for his mother to cook lunch. However, the boy was not certain exactly what her mother wanted him to buy. So he bought what he thought was the correct food, including fish, prawns and vegetables. Of course, they were not exactly what his mother would have bought.

That afternoon, the weirdest assortment of dishes was served on the table. No one spoke. My mother tried very hard to wipe the grin off my brother's face. Finally, everyone burst into laughter and started to "feast " a lunch that they will never forget.

What is the meaning of the underlined sentence?

Many thanks.
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She is trying to make him be serious and stop grinning - probably by telling him "Take that silly smile off your face! It's not funny."
Comments  
The sight of the food served caused much mirth among the family.

To wipe off the grin means to make outward signs of amusement on one's face disappear.
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 Feebs11's reply was promoted to an answer.
Feebs11She is trying to make him be serious and stop grinning - probably by telling him "Take that silly smile off your face! It's not funny."
Thanks, Feebs.