in the male gender, but it can also represent a mental state, maybe in girls too, I dunno....
2) "Yeah, I'm just BitTorrenting some more John Holmes..."
"Dude, I have a total hardon for what you're doing !!!!"
3) "Wow, that was a HARD hardon !!!!"
It signals to a woman that a guy is ready and able to have sex with her and eager to do so, which, if she finds him attractive, tends to kindle her interest. For that reason, it is a serious though common mistake for a guy to be embarrassed or self-conscious when he has an erection.