You can picture mettle as the material a person is made of. A piece of metal looks fine sitting on a table; only when you strike it do you find out whether it holds or bends. Mettle is that hidden hardness in a person, and a difficult situation is the strike that brings it into the open. That is why the word almost always arrives with testing attached to it.
From the word’s history
Mettle began simply as another spelling of metal, and from the 16th century English used it figuratively for the stuff a person is made of. The two spellings only came apart by meaning in the 18th century, leaving metal for the material and mettle for the person.
mettle 本來就是 metal 的另一種寫法。十六世紀開始，英文拿金屬來比喻人：這個人的材質好不好。到十八世紀，兩個拼法才依意思分家，metal 留給金屬，mettle 留給人。所以「敲下去才知道硬不硬」這個畫面，不是後來硬掰的比喻，而是這個字真正的來路。
Wiktionary, "mettle"
原始詞典資料4 筆記錄
以下是外部詞典記錄的用法，摘錄並精簡格式，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。
mettlenoun/ˈmɛtəl/
- 1
A quality of courage and endurance.
- By this Day and this Light, the fellow ha's mettell enough in his belly.
- Please read on and discover the issues in education that test the mettle of those who experience this world every day.
- 2
Good temperament and character.
- Your Maſter quits you: and for your ſeruice done him, / So much againſt the mettle of your ſex
- The arrival of this unexpected heir soon taking wind in the court, still makes good for the Sol, and keeps the court upon its mettle.
mettleadj/ˈmɛtəl/
- 1
Spirited, vigorous, stout-hearted.
- And then one afternoon in the hinder end of April came young Heriotside riding to the Skerburnfoot. His arm was healed, he had got him a fine new suit of green, and his horse was a mettle beast that well set off his figure.
這個拼法也被記錄為下列字的變化形：
摘錄自 英文維基詞典 English Wiktionary，已精簡格式；依 CC BY-SA 4.0 授權使用，改動後的內容同樣以此授權提供。