laugh up your sleeve
idiomPart ofsleeve
laugh up your sleeve — idiom
1. to be secretly amused about something while appearing serious on the outside.
1.idiom
Definition
to be secretly amused about something while appearing serious on the outside.
Example
When the boss tripped over the cable, Tyler laughed up his sleeve while helping him up with a straight face.
The students thought the teacher had not noticed their prank, but she was laughing up her sleeve the whole time.