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.

When you think something or someone is hot so you get a boner, but nobody else has a boner or thinks it is hot.

When she's "not in the mood".

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

The phase where one enjoys being alone and not in a relationship. After being single for many years enjoying the lone times and fearing the loss of that if they get into a commitment.

lonely
being by yourself
with no friends what so ever
pretty much having nobody next to you, to make fun of people with.

When you have to leave a group or be all alone because you got a boner and are embarrassed.