laugh up one's sleeve

Cultural  

1. To be secretly amused at something: “Arnie acted concerned over our plight, but we knew he was laughing up his sleeve.”

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laugh up one's sleeve Idioms  

1. Rejoice or exult secretly, hide one's amusement, as in When she tripped over her bridal train, her sister couldn't help laughing up her sleeve. This expression replaced the earlier laugh in one's sleeve, used by Richard Sheridan in The Rivals (1775): “'Tis false, sir, I know you are laughing in your sleeve.” The expression, which alludes to hiding one's laughter in big loose sleeves, was already a proverb in the mid-1500s.

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