When an insult is so surprising and uncalled for, it's the equivalent of slapping someone across the face. Kind of like verbal slap, the only difference being that mental slaps usually are an insult to intelligence, as verbal slaps can pertain to anything.
Person One - (Blah blah blah)
Person Two - ...You're an idiot!
Person Three - Mental Slap!
by More Awesome Than Thou November 26, 2005