laugh up your sleeve
US /læf ʌp jʊər sliːv/
UK /læf ʌp jʊər sliːv/
1.
to be secretly amused about something, often at someone else's expense
:
•
She tried to keep a straight face, but I could tell she was laughing up her sleeve.
•
The politician pretended to be concerned, but everyone knew he was laughing up his sleeve at the public's gullibility.