When you position your hand horizontally, grab one's face and proceed to swiping in a downward motion, palm down; Usually done when one is acting in a foolish manner. Can be done with wet hands for better results.
As a group of friends rock out to a 80's hair band genere of music, Mo sings the wrong refrain to a commonly known song and as a result to this Tom hits her up with a solid "facewash"
in a hockey fight when one player rubs the smelly ass plah of his hockey glove up and down the other players face, like he was washing his face, its one of the most annoying and aggrivatting things, not to mention degrading, a badass move.
ice hockey variation a facewash in an ice hockey game is a provocative move or an attempt at intimidation that may precede a fight where the aggressor performs the facewash wearing his or her hockey glove allowing the sweat-soaked (and often ubernasty smelling) leather palm of the glove to do the washing and make the point.