What does "死党" mean?
死 means 赴死 or 尽死力 ((worth) dying for / giving all one's strength).
党 means 朋党 (clique; cabal; faction).
死党 means &quot;pursue private ends&quot; at first,

[pursue private ends]∶ 为自己的朋党拼死竭力
案后将军朱博、 钜鹿太守 孙闳,故光禄大夫 陈咸( 王)立交通厚善,相与为腹心,有背心死党之信,欲相攀援,死而后已。——《汉书·瞿方进传》

And means &quot;sworn followers; diehard followers&quot; (with derogatory sence),

[sworn followers; diehard followers]∶效死的党羽
网罗死党

Nowadays it usually means &quot;best friend&quot;.

比喻情誼深厚的朋友。

Reference: 文化漫谈：为什么好朋友又称为“死党”？有何典故由来？