This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

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n. Courage and fortitude; spirit: troops who showed their mettle in combat.

n. Inherent quality of character and temperament.

idiom. on (one's) mettle Prepared to accept a challenge and do one's best.

n. A quality of endurance and courage.

n. Good temperament and character.

n. Metal; a metallic substance.

n. Substance or quality of temperament; spirit, esp. as regards honor, courage, fortitude, ardor, etc.; disposition; -- usually in a good sense.

n. Same as metal.

n. Physical or moral constitution; material.

n. Natural tempera me ut; specifically, a masculine and ardent temperament; spirit; courage; ardor; enthusiasm.

n. the courage to carry on

"I. i.96 (151,8) Of unimproved mettle hot and full] _Full of unimproved mettle_, is full of spirit not regulated or guided by knowledge or experience."

bravery    courage    courageousness    braveness   

to his mettle   

mettlesome   

disposition    energy    courage    heart    spirit   

Gretel    betel    kettle    metal    nettle    petal    resettle    settle    unsettle   

manliness    intrepidity    sprightliness    valour