to show someone that you do not respect them and do not think their ideas are important.
He asked them to put out their cigarettes but they just laughed in his face.
laugh in (one's) face
To react to what one has just said or done in a way that shows obvious ridicule and amusement, often by literally laughing in front of them. I pitched him my story, and he laughed in my face, right there in the elevator. It was humiliating.When Helen asked for a refund, the manager basically laughed in her face.I'd appreciate if you didn't laugh in my face every time I fall.
laugh in someone's face
show open contempt for someone by laughing rudely at them in their presence.