A person that has no life, friends, and especially girlfriend, who sits at home with nothing to do other than sit at home and beat off all the time.
"Where would I be without college? I wouldn't have friends, I wouldn't have a girlfriend, I would be the lonely boner that sits at home and jacks off all the time."
"Lonely Boner" refers to a virgin or someone who has been without sexual activity for a long time.
My friend Derrick is a lonely boner, or my brother Chad has a lonely boner.