A side friend is a person you are 'friends' with because your other friend is friends with them. So basically your just friends with them because your friend Becky is too. So when you, Becky and
that person are all talking, you guys are all considered friends, but if you were left alone with that side friend, it would be awkward to keep a convo going because you guys aren't really friends without Becky there.
You consider your side friend as one of
your friends, but deep down inside you know you guys don't really talk without someone thats both of
your friends.