At one time, metal and mettle were alternative spellings for the material or substance a thing was made of.
Classical Latin metallum had the meaning of mine or quarry as well as the substances obtained from them.
Metal is a hard, shiny, malleable material like gold, silver, or copper that is used in the manufacture of tools or artifacts. It’s shiny, malleable, and it conducts heat. The word metal can be used either as a mass noun or as a count noun:
Saucepan handles are usually made from wood or plastic because they don’t transfer heat as well as metal. (mass noun)
Metals play a critical role in the creation of contemporary jewelry by affecting appearance, durability and cost. (count noun)
Mettle derives from the same source as metal and was once used in the same way. Since about the 18th century, however, mettle has been reserved for figurative use. It refers to what a person is “made of.” Mettle includes character, disposition, and temperament. It is often used with the verbs test, prove, and show, particularly in sports writing:
Bruins’ Marchand shows his mettle
Djokovic must prove his mettle against Nadal on clay
Young runners test their mettle during Hy-Vee runs
Kelso showed his mettle and won by three-quarters of a length.
Shakespeare uses the word in Macbeth’s sexist compliment to his wife after she berates him for hesitating to kill Duncan:
Bring forth men-children only;
For thy undaunted mettle should compose
Nothing but males. –Macbeth, I, vii, 72-74.
The adjective mettlesome describes a high-spirited, vivacious, lively person:
In The Taming of the Shrew, Kate is Baptista’s mettlesome elder daughter.
Mettlesome can also mean courageous: “Lü Kuang is fresh from his victories in the west, and his soldiers are vigorous and mettlesome.” –The Art of War, XI.
Applied to an animal, especially a horse, it means “lively, eager, spirited, frisky.” It can also be applied to an event:
[The tennis players] delivered a mettlesome performance from start to finish, bringing the ultimate title under their names.
Mettlesome from the noun mettle is not to be confused with meddlesome, the adjective that goes with the verb meddle, “to interfere.”
Hillsdale College Chief Remark Pricks Meddlesome Bureaucrats
How to Handle Your Meddlesome In Laws
Meddle comes from a word meaning “to mix.” Meddlesome people mix in where they are not wanted. Mettlesome people are often fun to know.