What people now call getting “double-crossed” was previously just called getting “crossed,” that is, deceived by another. The term “double cross” appeared in 1834 to describe when an individual simultaneously deceives two different parties after convincing each that he is their ally in cheating the other. When the scheme plays out, two different parties find themselves betrayed, so there has been a “double cross.” However, when most modern people use the term, they don’t mean this complicated three-party plot, just a straightforward one-person-cheating-another scenario.
The term “sold down the river” has ugly roots in American slavery. During the years of the 1800s when slavery was legal, the city of Louisville, Kentucky housed one of the nation’s largest slave markets. From there, many purchased slaves were sent further south along the Ohio and Mississippi Rivers to cotton plantations, where they often endured brutal labor and mistreatment which frequently proved fatal. Hence, getting someone “sold down the river” came to mean a betrayal so complete it might lead to death.
The names of some historical betrayers have become synonymous with “traitor.” Judas betrayed Jesus to the Romans, Brutus helped assassinate his friend and emperor Julius Caesar, and Benedict Arnold sold out his native United States to the British.