Mochi is a small Japanese rice cake and can be used to describe someone who is small and cute. To call someone mochi sexy is to say that he/she is both cute and sexy at the same time. The phrase - known in the K-pop community - originated with boy group member Park Jimin, who exudes cuteness and sexiness, sometimes at the same time.
Person A: how can he be this sexy when he was just giggling adorably five seconds ago?
Person B: he's mochi sexy. The best aesthetic.
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.