Picture an army pulling back. One small group does not go with it. They stay at the back, facing the people who are chasing, so everyone else can get clear. They are not trying to win — they are buying hours. Every use of rearguard in English comes back to that group and that job.
The word is rear + guard, and its older shape says the same thing: Old French rereguarde, from rere 'behind' and guarde 'guard', Middle English rier garde. The name has always been simply 'the guard at the back'.
這個字就是 rear（後面）加 guard（守衛）。更早的形狀說的也是同一件事：古法文 rereguarde，rere 是「在後面」，guarde 是「守衛」，中古英語寫作 rier garde。從頭到尾，它的名字就是「後面那組守的人」。
Wiktionary, entry 'rearguard' (source_ref kaikki:3f6f69a190036ae4:1028537394:12215), supplied in the evidence packet
From the picture
邊退邊守的那場仗
從那批人身上長出一個說法：他們打的仗，就叫 a rearguard action。所以這個家族講的不是人，是那種「已經在撤、還要邊退邊打」的仗。
noun · C1
When an army fights a rearguard action, it goes on defending itself at the back as it pulls away, holding off the force chasing it instead of trying to win.
殿後之戰
noun · C1
The part of an army that stays at the back while the rest of it moves away, holding off the force that is following so the others can get clear.
殿後部隊
noun · C1
In writing about football, the players who defend — spoken of as one group at the back of the team.
後防線