“Yucking someone’s yum” could be defined as being overly negative about something that someone else is fond of, in front of them. I’ve yucked people’s yum before without realizing it. Especially when it comes to pro sports, which I don’t think I’ll ever fully understand. I know better now. Yucking the yum is the type of activity that can cause long-term shame (of the yummer) in exchange for short-term satisfaction (of the yucker).