A trivial/small action done by someone that is the final and concluding act in a relationship/confrontation or event. It is also usually the tipping point in a confrontation that had been building ie. final straw
Girl: " Stop flicking me"
Guy: Chill, jesus.
Girl: "OKAY, thats enough!! You've poked the yoke, Its over. You have been constantly annoying me, way to clingy, you whistle off tune, and I think we should stop seeing each other. That flick was the last straw.
Guy: Uhhh, kay.