Today’s idiom is a bit violent, maybe a bit bloody, but very descriptive. All the best idioms are, so pay attention and enjoy the gossip (Klatsch) :
„Why is Karl packing up his things? Has he been fired?“
„You haven’t heard? He’s running his own business on the side… Apparently, he and Michelle started back when we lost that big account in Hartford, three years ago. They picked up the business for a much lower price and have been delivering to the old customer ever since!“
„So how did the news get out?“
„Well, some people say that he was taking too much money, and Michelle got angry.“
„So she threw him under the bus?“
„It appears so…“
In this dialogue, two employees have apparently been caught doing business with former customers. For whatever reason, Michelle told on (petzen) her colleague Karl. When we tell on someone, or rat them out (another good idiom), we can also say that we’ve thrown them under the bus (jemandem in den Rücken fallen). You can imagine two good friends, waiting for the bus. The bus pulls up, and the one friend wants to get in the bus before the other and therefore throws the other friend into the street… maybe even to die! Not a pleasant image!
The origins of this idiom are unclear and even disputed (umstritten). Some give Cyndie Lauper the credit; others claim it comes from emplyees at a famous Mexican restaurant in Denver, Colorado in the 1980s; and still others say the phrase originally comes from the UK political scene even before that. A quick Google search will provide you with the details.