Whenever someone makes a joke that they say "is really funny" before they tell it, and it really isn't funny so there is a big long drawn out silence while everyone stares at the kid wondering what the fuck he was thinking when he told it. Derived from the expression: "The only thing that could've made that funny was if you were wearing a sombrero".
Kid #1: Oh my god, this is so funny! I went to the store, and an old lady rung me up, and then asked me for my drivers license! haha!
Kid #2: Sombrero moment, dude.