Don't play dumb.=Don't pretend you don't know. 别装傻；别装什么都不知道
例句：
Don't play dumb,I know you ate the last piece of cake.
别装傻了，我知道你吃了最后一块蛋糕。
Don't play dumb with me,I know you took the money.
别跟我装傻了，我知道你拿了钱。
A:I don't know anything about that.
我什么都不知道。
B:Don't play dumb.
别装傻了。