This is a nickname given to someone who is cynical, hates fun, and hates it when other people have fun. Originally derived from the stereotypical view of an angry grandmother who thinks her grandkids shouldn't be allowed to play and have fun.
Don't be such a Grandma Kill Joy, come to the party! It will be fun!
Girl: Yesh, finally Full Metal Alchemist. Oh how I've longed for you. *kisses television*
Mom: No FULL METAL ALCHEMIST. Japan is evil.
Girl: Geez Mom, you're such a kill-joy!
Girl: Yesh, No school because of snow!
Weather Man: Looks like school is back in session due to the sudden elevation of tempurature.
Girl: Geez Mr. Meteorologist Man Guy, you're such a kill-joy.
A guy who, due to ego or low intelligence, refuses to acknowledge that he didn’t get understand the joke, so instead resorts to insisting that there was never a joke in the first place. He will often criticize the intelligence of the person telling the joke on the basis that they were serious.
“Make sure not to use satire around that guy! He’s Dunning-Kruger’s Kill-Joy, he’ll trap you in an argument you never actually made!”