Akin to asshat, fucktard, twit and various other condescending names for someone who behaves badly.
Can also be used as "douchemoosery", such as "He was so full of douchemoosery that we had to go hang somewhere else."
"I thought Susie looked great, but Bob was being a douchemoose and told her she looked awful."
"If you can't say anything nice, don't be a douchemoose. Just don't talk."