Being "Hit like Holyfield" means that the girl sleeps around a lot. She has been "hit" by many different guys.
The fighters that fought Holyfield typically took many punches and the fights lasted the full 12 rounds. There were "hit" many, many times. A girl that has been "hit like Holyfied" is probably not safe and also has STDs.
Brad: Damn, that girl over there is a dime.
Homewood: Hell yeah, but she has been hit like Holyfield so you know you will be at the free clinic on Monday wondering what is dripping from your cock if you settle for her tonight.
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.