A hot charlie is anything you want it to be. Because of the way it sounds, most people (especially girlfriends) associate it with something sexual (i.e. Dirty Sanchez). Simply ask for a hot charlie, and the other person must determine what his/her definition of it actually is.
Me: Hey, I'm going to ask my girlfriend if she wants to give me a hot charlie tomorrow night after the movies.
When you're hitting a girl from behind, leave for a second to somehow heat up your dong, and then return to stick it in her butt. It's customary to yell "Hot Charleys for everyone!" right after this.