To experience a drastic change in one's emotions or fortunes, especially from happiness to a negative emotion, often in a short span of time. A: "I thought your mother was happy that we're getting married." B: "Me too, but now, she's laughing out the other side of her mouth, and I get an angry phone call from her every day." They'll be laughing out of the other side of their mouths when they realize how much they'll have to pay in late fees.