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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Browse entries near laugh up one's sleeve
laugh out of the other side of one's face
laugh out of the other side of one's mouth
laugh out of the wrong side of one's face
laugh out of the wrong side of one's mouth
laugh track
laugh up one's sleeve (idioms)
Laugh, and the world laughs with you; weep, and you weep alone
laughable
laughable (synonyms)
laughableness