To double-cross someone is to cheat them after they thought you were on the same side.

To double-cross someone double-cross 某人， is就是 after they thought you were on the same side在他以為你們是同一邊之後， to cheat them騙了他。

The extra sting is this: you had already agreed to do something together — often something a bit shady — and then you turn around and cheat that partner. So you are not only breaking a normal promise; you are betraying the person in the plan with you.

The extra sting is this多出來的那根刺在這裡： you had already agreed你們本來已經講好 to do something together要一起做一件事—— often something a bit shady常常還是件不太光彩的事—— and then you turn around然後你回過頭來 and cheat that partner騙了那個夥伴。 So所以 you are not only breaking你不只是毀了 a normal promise一個普通的承諾； you are betraying你是背叛了 in the plan with you和你同在一個計畫裡的 the person那個人。

People also stretch it to any big sell-out: a friend who tells on you, a teammate who secretly helps the other side. It is informal, a bit dramatic, and very at home in crime stories and spy films, though everyday talk uses it too when someone feels stabbed in the back.

People also stretch it to人們也把它延伸到 any big sell-out任何嚴重的出賣： who tells on you去告你狀的 a friend朋友、 who secretly helps the other side偷偷幫對面的 a teammate隊友。 It is informal它不正式、 a bit dramatic帶點戲劇性， and very at home最自在的地方 in crime stories and spy films是犯罪故事和間諜片， though不過 when someone feels stabbed in the back當有人覺得被人從背後捅了一刀時， everyday talk uses it too日常對話也會用它。

As a noun, a double-cross is the act itself: “It was a double-cross.” The old thieves’ slang helps: on the cross meant dishonest (the opposite of straight or square). A crook who then cheats his own crooked partners is crossing the crossers — a double-cross. First recorded in that world in 1834.

As a noun當名詞用時， a double-cross is一個 double-cross 就是 the act itself那個行為本身： “It was a double-cross.”那是一場出賣。 The old thieves’ slang helps早年竊賊的黑話能幫上忙： on the cross meant的意思曾是 dishonest不老實 (of straight or square是 straight 或 square 的 the opposite相反)。 who then cheats his own crooked partners連自己那群不老實同夥都騙的 A crook騙子， is crossing the crossers是在騙那些騙人的人—— a double-cross這就是 double-cross。 in that world在那個圈子裡 First recorded最早的紀錄 in 1834是在 1834 年。