The words "swipe left" was first used to reject future spouses, potential dating partners, and/or Hillary Clinton on the infamous dating app, Tinder. The phrase has devolped over time and now describes anything remotely ugly or detestable.
In the case of an undesirable erection or a narb (no apparent reason boner), the meneuver done to hide such displeasures from the public eye. The move consists of glancing left, glancing right, then swiping the erection up, to be held by the elastic or waste of the pants.