As described in Rachael Yamagata's song "Sidedish Friend". A friend who you don't want to always hang out with you, but you want them when you call them. Someone to have as a friend only when it's good for you.
I don't want to hang out with you all the time, okay? You're not my friend, you're just a sidedish friend.
A relationship status in which one performs all of the functions and roles of a boyfriend without any reciprocation of the emotional, romantic, spiritual, and physical intimacy that comes with a healthy relationship
Someone that gets used by another person when that other person has another boy/girl friend.
Johnny: Man, i got so closed to being laid last night by Suzy!
Bob: Shut up man, you are just her sidedish, her real boyfriend took care of that for you.