A guy who can fill in for you when your girl wants to have sex and, for whatever reason, you are unable to handle the deed. A guy who will loan you his bone so your chick doesn't trip, and then will leave when the job is done.
A boner or erection that occurs in one's home, or in any other place alone. This includes when they are looking at porn or reading/thinking about something erotic. A Loner Boner can also occur when somebody unattractive is around, such as a parent. However, if you are with a girl (or boy, depending on your gender preference) and you get a boner, this is not considered a Loner Boner.
Jeff: Dude, I just got a total Loner Boner last night.
Mike: What did you do?
Jeff: What every civilized man would do! Took advantage of it!
A loner boner is when you get a boner/masturbate to fictional characters because you're lonely and don't have anyone in real life
Guy 1 "Bro, I was playing Persona 5the other day, and got a boner when i saw Futaba."
Guy 2 "Damn, i thought you said you had a girlfriend."
Guy 1 "I do."
Guy 2 "Then why did you get a loner boner?"
Guy 1 "¯\_(ツ)_/¯"