When the skidmarks on your underwear dry out and start flaking off, turning into dust. It happens when people do not wipe up well after going to the bathroom, and when they do not change their underwear frequently.
I haven't changed my underwear in six weeks and now I think I'm starting to see some crust dust coming out of my pant leg.
While our lifespan is the number of years we will live, our healthspan is the number of healthy years we will have and our sexspan is is the number of years we will continue to be healthy enough to continue having an active sex life.
A lighthearted reaction upon receiving disappointing news of some sort. Usually something that isn't a particularly a big deal, esp. when it primarily affects someone else - sort of a mild schadenfreude. Sometimes doubled or tripled, and possibly said in a sing-songy voice for added comedic effect to imitate a muted trumpet, which is how the meme originated.