When someone is commiting an act of douchebaggery
and the people around him/her make a "d" shape with their hands and say "D um' up!... DOUCHE!"
Often followed by the douchebag's own form of retaliation
(example: "That's what she said", "Screw you", ect.)
Sometimes the "...DOUCHE" ending can be subtracted, leaving the douchebag confused and angry.