An inbox can be pictured as the one spot where things that come IN to you are left to wait. Nobody asks you first — the letters, the messages, the jobs simply land there. They sit in that spot until you pick them up and do something with them, and then they leave. That is why the same word covers a shallow tray of paper on a desk and a list of unread messages on a screen: both are the place where incoming things queue for you.
「inbox」可以想成「東西進來以後，暫時堆放的那一個地方」。信、訊息、事情不會先問過你，它們就是自己掉進去。掉進去之後就停在那裡等。等你去拿、去處理，處理完才離開。所以不管是桌上那個裝紙的淺盤，還是螢幕上那串未讀訊息，用的都是同一個字：它們都是「進來的東西排隊等你」的位置。
From the word’s history
Wiktionary records the word as simply in + box: the box for what comes in. The modern core is still exactly that.
資料來源記的字源很直接：in（進來）＋ box（盒子），就是「裝進來的東西的盒子」。今天不管是紙盤還是螢幕上的資料夾，核心畫面都還是同一個。
Wiktionary via packet source_ref kaikki:3f6f69a190036ae4:1373991324 (etymology_text: From in + box.)
登入就可以看每一支怎麼連回核心畫面。