be laughing up your sleeve

be laughing up (one's) sleeve

To be trying to hide that one is laughing at someone else. It seems like the popular kids are laughing up their sleeves every time I walk by. It's so infuriating.
See also: laugh, sleeve, up
Farlex Dictionary of Idioms.

be laughing up your sleeve

OLD-FASHIONED
If someone is laughing up their sleeve, they are secretly amused by something, often something silly that someone else has done. They must be laughing up their sleeves at our stupidity. Note: The image here is of someone trying to hide the fact that they are laughing by putting their hand or arm in front of their mouth.
See also: laugh, sleeve, up
Collins COBUILD Idioms Dictionary, 3rd ed.
See also:
  • be laughing up (one's) sleeve
  • sleeve
  • bust out laughing
  • rolling on the floor laughing
  • be laughing (one's) head off
  • be laughing your head off
  • laughing in the aisles
  • burst out laughing