Nick: Dude, that meeting was brutal...
Mark: Oh man, I know, I totally headdesked an hour into it.
Nick: yeah, the partner saw you, too. good thing he almost headdesked or you'd be busted.
When a person becomes so overcome with fangirl delight that they can no longer support the weight of their own head, and it hits a desk or desk-like object.
Verb. The act of slamming your head against your desk or any hard object in front of you. Often mean to vent frustration, show frustration, or to demonstrate how frustrated you are in a conversation.
P.S. Everyone knows it's spelled with one 'D' not two.