1. slang To decline or reject someone or something in an app. The phrase was popularized by the dating app Tinder, in which users "swipe left" on the screen to reject a potential mate. That girl is pretty—I'm only swiping left because I'm not into blondes. Ugh, this guy's has no sense of humor—swipe left!
2. slang By extension, to reject or decline something. Studying instead of a night out? Definitely swiping left on that.