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 convinces two separate parties that that he will help them cheat the other. When the scheme plays out, both of the other 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. When the 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 often-brutal treatment frequently proved fatal. Hence, getting someone “sold down the river” meant 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.
The word “turncoat” is born from the ancient practice of wearing a badge of your allegiance on your coat that could be hidden if you turned your coat inside out.
Treason is the only crime defined in the US constitution.
To “drop a dime” on someone comes from the practice of making a pay phone call to the police to inform on them. These were also practical, since short, unexpected pay phone calls couldn’t be traced in these early telephone days.
Since the late 1800’s, the word “snitch” meant nose, and since nosy people are involved in others’ business, this soon came to also mean an informer.