A girl who is easy. Like a scoring a goal on an empty net
"Did you see that chick i hooked up with last night!"
"Yea man, i bet you had to spit some game to hit that!"
"Nah man she was an empty netter"
A humorous comment that occurs when somebody says something that lends itself to a great joke, as in an empty net goal in hockey. An empty netter is often a sex-related comment.
When my girlfriend said I never knew what would come out of her mouth, it was an empty netter for me to say that I know what goes into her mouth.