在中国古代，“死党”是为了某人或组织竭尽全力甚至付出生命的人。到如今，“死党”的意思是跟你关系很好，可以无话不谈，无所顾忌、相互依靠、亲密无间的朋友。
In ancient China，“ sǐdǎng” was a person who would spare no effort and even sacrifice his life for another person or group. Today, “ sǐdǎng” means your close friends, ones from whom you keep no secrets and towards whom you have no misgivings. These are friends who can rely on each other.

Sentences:

Dàxué sìnián de xiāngchǔràng wǒmen liǎnggè chéngle sǐdǎng.
1. 大学四年的相处让我们两个成了死党。
We became best friends after spending four years of college together.

Tā zhǐyǒu gēn sǐdǎng zài yīqǐshí cáihuì wúhuàbùtán.
2. 他只有跟死党在一起时才会无话不谈。
Only when he is with his best friend does he speak without holding anything back.