The idiom refers to something not to be taken seriously. Something spoken might be the opposite of what a person really meant. ‘That was very good’ might be said when something was not very good. The idiom comes from visual clues like pushing the tongue against the inside of the cheek or biting the tongue in order to not laugh.
說反話、口是心非
這慣用語說的是別看得太認真，有些事情說出來並不一定是內心所想的、或許是完全相反意思。