To cast a person in an unfavorable light with others; to take action or make statements intended to put another person at a disadvantage.
Origin: A Boston radio station manager coined the term circa 1987-88 when canceling a radio network's services on his music-oriented FM station, stating that he was going to put the network "under the bus." The term was picked up by staff members to describe conduct in which one person would try to gain an advantage in company politics by speaking ill of, or doing something to reflect disfavorably on, another. In this context, it generally meant something that was a combination of sneaky, subtle and vicious. The phrase crept into on-air talk. In time, the radio station's owner acquired a sports-oriented station whose employees picked up the phrase and eventually began using it on highly-rated programs.
Joe really threw Sally under the bus in the meeting today. She wasn't there and he said the company would have won the Simpson account if Sally hadn't gotten drunk at the lunch meeting.