be laughing up (your) sleeve

to laugh at someone secretly, often in an unkind way.

He persuaded people to believe in him and all the time he was laughing up his sleeve at them. (often + at)

• lend an ear to
• lend an ear to someone
• lend an ear to something
• lend itself to
• lend itself to something
• lend someone a hand
• leopard can't change its spots.
• leopard doesn't change its spots.
• lesser of two evils
• less than
• less than something
• let alone
• let alone do something
• let be
• let bygones be bygones
• Let bygones be bygones.
• let down
• let fly
• let fly something
• let fly with
• let fly with something
• let go
• let go of
• let go of someone
• let go of something