あの人（ひと）は 二枚舌（にまいじた）を 使（つか）う。
舌（した） is a tongue.
a. He has two tongues.
b. He has a split tongue like a fork.
c. He is two-faced.
d. He uses his tongue like a fork.
The answer is c. He says one thing to one person and says a different thing to another. So you may be able to call him a liar also.
