郵件的出入口
這一支是字面上的用法：真的裝信的那個開口或箱子。核心畫面在這裡不用轉彎。
noun · A2
A narrow slot in a front door, or a small box fixed outside a house, where letters and small packages are delivered.
信箱／投信口
核心畫面
You can think of letterbox as a cut for letters: a long, straight-sided opening, or the small box behind it, where something is dropped in and waits to be picked up. Nearly everything the word does seems to grow out of that shape and that job. The slot in a front door and the box in the street are the plain cases. A wide film sitting in a long thin band across a screen borrows the shape. A box hidden on a hill, holding a notebook for the next walker to open, borrows the job.
「letterbox」可以想成「為了信而開的一個長方形」。一條窄窄的、四邊很直的開口。開口後面常常還有一個小箱子。東…更多收起
「letterbox」可以想成「為了信而開的一個長方形」。一條窄窄的、四邊很直的開口。開口後面常常還有一個小箱子。東西被投進去，停在那裡，等人來拿。這個字的各種用法，大多可以想成從這個形狀跟這個工作長出來。門上的縫是這樣。街上的郵筒也是這樣。寬螢幕電影被放進螢幕中間那條長長的帶子，看起來還是同一個長方形。山上藏著等人打開的小盒子，做的則是同一件事：東西放進去，等下一個人。From the word’s history
The word is transparent: letter + box, a box for letters. The modern uses still show that rectangle and that waiting.
這個字的來歷就寫在字面上：letter（信）加 box（箱子），也就是「裝信的箱子」。今天不管是門上的縫、街上的郵筒，還是螢幕上那條長方形的畫面，都還看得到當初那個形狀。
packet evidence: Wiktionary etymology_text for letterbox, 'From letter + box.' (source_ref kaikki:3f6f69a190036ae4:1036155001:49356)
Three branches
像投信口的長方形開口
這一支借走的是形狀，不是郵件。只要一個開口又長又扁、四邊很直，英文就可以叫它 letterbox。
noun · C2
In climbing: a rectangular hole or slot in rock, ice or snow, sometimes cut with an axe, that a climber can use as a hold or a step.
岩壁或冰面上的方形凹口
藏起來等人打開的盒子
這一支借走的是「箱子」跟「放進去、等下一個人來」的那件事，形狀反而不重要。
noun · C1
A small box hidden outdoors, holding a notebook and a rubber stamp, which people look for by following written clues, as a game.
藏在戶外的尋寶盒
原始詞典資料7 筆記錄
以下是外部詞典記錄的用法，原文照列，我們還沒有寫成教學內容。上面的說明才是我們自己寫的；有些用法可能上面已經講過了。
letterboxnoun
- 1
A box into which mail is put.
- 2
A box into which mail is put.
- A wad of junk mail was pushed through the letterbox in the front door and dropped onto the mat.
- 3
A hidden container that holds a logbook and rubber stamp, found by following clues as a form of recreation.
- 4
hobbieslifestylemountaineeringA rectangular hole or slot.
- His strength was running out fast: one more good letterbox was all that he could conceive himself able to make. He made it, hacking away with slow, painful strokes, his axe-handle slippery with his sweat.
letterboxadj
- 1
broadcastingfilmmediaTransferred to home video formats while preserving the original aspect ratio, having black bars above and below the picture area.
letterboxverb
- 1
broadcastingfilmmediaTo transfer a widescreen motion picture to home video formats while preserving the original aspect ratio, with the placing of black bars above and below the picture area.
- 2
To hunt for letterboxes (containers with logbook and rubber stamp) by following clues.
資料來源：英文維基詞典 English Wiktionary（CC BY-SA 4.0）。 原文照列，未經改寫。