Well I guess the "achievement of erection"
in the male gender, but it can also represent a mental state, maybe in girls too, I dunno....
1) "Well, first ya git 'er down on 'er hands and KNEES.... Then ya reach around and grab a handful o' BOOB... Then see if ya don't get a hardon !!!
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 !!!!"