When someone is double crossed, it means that a partner backed out of a previously-reached agreement. People can double cross each other in a variety of creative ways, ranging from reneging on an agreement outright to doing the opposite of what was agreed upon. As a general rule, this term is associated with treachery, since the assumption is that the agreement was reached in good faith, and the decision to go back on the agreement is a form of betrayal.
Some people consider a double cross a form of backstabbing, arguing that when an agreement is reached with someone, both parties are expected to honor the agreement and trust each other. Double crossing someone betrays that trust, making it difficult to reach agreements in the future because of the constant threat of a repeat episode.
There are a number of explanations for the origins of the phrase. One of the most likely comes from the early 1800s, when a “cross” was a fixed horse race. The use of the term “cross” to describe some sort of agreed-upon criminal activity was very common in this period. If a horse who was supposed to lose won, the owner would be accused of a “double cross” which violated the original agreement. In addition to being supported by other uses of the word “cross” for a fixed deal, this explanation appeals to many word historians because of the clarity and the date, which can be linked with the first uses of “double cross” as a slang term.
Others have suggested that the term may originate in the practices of thief-takers in 19th century England. A thief-taker was someone who would turn in suspected criminals and thieves for a bounty. Many of them accepted payments from thieves who wanted to stay out of jail, and legend has it that they marked their lists with crosses, using a double cross to mark someone who didn't pay up. This explanation is not entirely satisfactory, however, although it is less spurious than the theory that people would mark an agreement with two crosses when they were illiterate to reflect the idea that they had no intention of keeping it.
Some people also associate the term with the Double Cross System, a tactic used by British intelligence in the Second World War. This system recruited German agents to feed misinformation back to the German government, and according to legend, the spies met in Room 20, or XX in Roman numerals. However, the term “double cross” predates the Second World War by at least 100 years, and spies do not usually make a habit of meeting in large groups in the same room on a regular basis.