If you cross someone, you’re cheating him.
A double-cross means you are cheating both your employer and the one you’ve been hired to deceive.
In the 1800s, Thackeray described a fixed horse race in Vanity Fair where the jockey who was prearranged to lose was instead allowed to win, costing the gamblers a fortune.
Because the fixer had crossed or cheated both parties for a huge profit, the win was called a “double-cross.”
