The male best friend of a female. They do everything together but they're not in a relationship.
"Hey what's Liz doing tonight?"
"She's going out to that new bar with her 'Pseudo Boyfriend' and wants to meet up with us later"
someone that has never been your boyfriend/girlfriend but you know deep down they would be perfect for you so you resort to being good friends. Sometimes living in different states influence a pseudo sitch.
"Monica don't forget I'm your pseudo boyfriend"
Monica: "I know you are ;)"