My first impression to this word is something like "enemy" or "enemies", because the letter "死" is used and it means death.
But when I do some search, it means "sworn followers"?
I really doubt at it. Can anyone explain?
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 "pursue private ends" 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 "sworn followers; diehard followers" (with derogatory sence),
[sworn followers; diehard followers]∶效死的党羽
Nowadays it usually means "best friend".
It means best friends or very close friends. No bad meaning involved.
it means sb is your follower. also same with "老铁". u guys know each other very well.