The act of fucking people on the side while in a horrible long distance relationship. Since people cannot cope with long distance relationships they need the additional sexual satisfaction of having sex with other people. The long distance partner cannot possibly satisfy the other through the use of phone or web sex, so additional partners are required to quench this desire.
D: So how's your boyfriend in Vancouver doing?
J: So fucking bad, thank god I'm sidefucking.
D: Oh I know what you mean, sidefucking is essential. Like when I was dating that chick in Utah.