The term "back-door friend" is not a commonly used phrase, and it does not have a standard definition. However, it could be interpreted as a friend who is not as close as a "front-door friend" and who might only be allowed to enter your home through the back door or a side entrance.

In this sense, a "back-door friend" might be someone who you are less familiar with or less comfortable around, or someone who you don't trust as much as a "front-door friend." The term could also be used to describe someone who is not always welcome in your home, or who you don't enjoy spending time with as much as other friends.

It's worth noting that the term "back-door friend" is not a widely recognized or established term, and its meaning may vary depending on the context and the person using it. Additionally, using terms like "front-door friend" and "back-door friend" to categorize and rank the importance of friendships can be potentially harmful, as it can perpetuate exclusivity and social hierarchies.