The act of ignoring a phone call from an annoying caller after very few rings, aka hitting the "f-you" button. The term is rooted out of the fact that many popular mobile phone models have a sidebar button to ignore/silence calls.
"Dude, I called you last night and you sidebarred me after 2 rings. Wtf?"
"Omg Kelly... this douchebag just called me and I sidebarred him for the third time this week. Why can't he get that I'm just not that into him?!"