laugh up your sleeve
laugh up (one's) sleeve
To try to hide that one is laughing at someone else. Ron may seem like a nice guy, but he laughs up his sleeve whenever he's around someone he dislikes. The popular kids laugh up their sleeves every time I walk by—it's so infuriating.
laugh up your sleevebe secretly or inwardly amused.
The use of up in this expression is a relatively recent development; the phrase dates from the mid 16th century in the form laugh in your sleeve .