To double-cross means to betray someone who trusted you, especially after pretending to work with them. When someone double-crosses you, they break a promise or agreement in a sneaky way, often to help themselves at your expense.

Imagine two students agree to study together for a test, but one secretly plans to use the other's notes and then claim all the credit for a group project. That's a double-cross: acting like a partner while actually planning betrayal.

A double-cross is worse than simply breaking a promise because it involves deliberate deception. The person doing the double-crossing usually gains the other person's trust first, making the betrayal more shocking and hurtful. In spy novels and adventure stories, a character might double-cross their partners in crime by taking all the stolen treasure for themselves, or a secret agent might double-cross the villain by pretending to help while actually working for the good guys.

The phrase can be used as a noun too: “The villain's double-cross surprised everyone.” When someone gets double-crossed, they learn a hard lesson about trust and loyalty, and they may become more careful about who they rely on.