Sometimes a normal reaction just isn't enough. What you have witnessed was too dumb, and too idiotic to really happen out here in the real world. Someone who is an adult, with a job, and presumable life and family decided to say or do something so outlandish that you have no real reaction. What is left to do? There is no reasoning, so debating, no steps forward to solving this issue of mindlessness. Where would you even begin? At this point, the only possible appropriate reaction is to bring that palm right up to that face and facepalm yourself.

Not every problem in this world can be solved by you. Some of them you have to just let be, and hope someone else does or doesn't fix it for you. There is only so much time in a day, a year, and a life, and you can't clog it up trying to reason with unreasonable people. What you can do bring your face to your palm to help you deal with all that you can't actually deal with.

From r/facepalm.