Bolshevik can be pictured as a party badge from one year in Russian history. It first pinned on a real person: a member of the group that took power in 1917. A badge like that is easy to take off the original owner and pin on somebody else, and that is what the second use does — someone throws the name at a person they disagree with, meaning it as an insult rather than as a fact.
Borrowed from Russian большевик, from большинство, meaning "majority". The name was taken by one side of a split as the larger group, and it then hardened into the party's name, which is the historical sense English borrowed.
這個字借自俄文 большевик，來自 большинство（多數）。名字原本是分裂時自稱人數較多的那一邊用的，後來固定成整個黨派的稱呼。英文借進來的，就是這個歷史上的用法。
Wiktionary entry supplied in this packet (source_ref kaikki:3f6f69a190036ae4:1063213484), etymology_text: "Borrowed from Russian большеви́к (bolʹševík), from большинство́ (bolʹšinstvó, 'majority')."
From the picture
被拿來當標籤丟的用法
一個具體黨派的名字，被放大成「所有立場偏左的人」的統稱，還帶上敵意，就成了這個用法。徽章還是同一枚，只是別的人幫你別上去的。
noun · C1
An insulting name for a communist, or for anyone whose political ideas seem far to the left. The source marks this use as old-fashioned, so it belongs mainly to the past.
赤色分子（貶、舊）
noun · C1
A member or supporter of the communist party that took power in Russia in the revolution of 1917. It is a history word: it names people of that time and the party they belonged to.
布爾什維克