double cross
1. noun An act of duplicitous betrayal or swindling, especially of a friend, ally, or colleague. Originally a sporting term in which a "cross" referred to an event that had been fixed by the participants to fail; a "double cross" happened when one participant secretly backed out of that arrangement and went on to win the event. Double crosses happen all the time in politics, with politicians making promises to each other behind closed doors and reneging upon them down the road. Jonathan's double cross ended up costing our company millions of dollars of wasted research and development.
2. verb To betray or cheat someone in a duplicitous manner, especially by going back on a previously agreed upon arrangement. We've been double-crossed, fellas, so keep your eyes open for the cops. John and I spent years developing the product together, but he double-crossed me once it was finished and got a patent for it under his name alone.