A douchehorse is someone who attempts to conquer Japan by sending a fleet of shitty riverboats to sea, then gets all surprised when every single boat sinks in a storm, resulting in the highest number of at-sea casualties in history.
douche + asshole. thereby, making the person in question, more of a douche and an asshole at the same time. generally used with people who just drive you crazy and calling them anything else just seems too little. it was invented for frank lampard.