They take it already upon their salvation, that though I be but Prince of Wales, yet I am the king of courtesy; and tell me flatly I am no proud Jack, like Falstaff, but a corinthian, a lad of mettle, a good boy,—by the Lord, so they call me;—and, when I am King of England, I shall command all the good lads in Eastcheap.
O, this boy Lends mettle to us all!
I wonder much, Being men of such great leading as you are, That you foresee not what impediments Drag back our expedition: certain Horse Of my cousin Vernon’s are not yet come up: Your uncle Worcester’s Horse came but to-day; And now their pride and mettle is asleep, Their courage with hard labour tame and dull, That not a horse is half the half himself.
There are no more uses of "mettle" in the play.
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The Trials are a test of my mettle.
Sabaa Tahir -- An Ember in the Ashes
But I will say, too, that for every man who showed no fear or hesitance, there were three or four or five others whose mettle was as unashamedly wan and mortal as yours or mine.