Derived from the Comedie de L'arte, the facepalm gesture signified a change of humour based on a realization, a change of mask, to put on a new face.
The gesture became a standard trope of grief or weeping in theatre but when used comedically, indicated a shared misfortune. Imported into Vaudeville, the Three Stooges' innovated the gesture as a surrealistic form of violence, indicating surprise, fear and disdain.
The completed gesture was a stock expression in newspaper comics and used in animated films, exaggerating a sense of mock tragedy or comic disbelief.

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The act of dropping one's face / forehead into one's hand. Usually accompanied by a "thunk" or a cr a cry of "D'oh!"

Usually written between asteriks in online conversation, to demonstrate an action.

Similar to *headdesk*

The only logical answer to a stupid question or statement.

What I do whenever I read Urban Dictionary.

to hit one's own forehead with your hand/palm, and drag it down one's face. Most often done in frustration or agitation.
Sometimes forcefully done to others for humour purposes.

When someone is so stupid that it makes you speechless, and the only thing you can do is shake your head or facepalm.

A spontaneous reaction to an amazingly stupid statement, where the face of the listener meets with his palm in a smacking manner