As soon as Rocinante found himself free, though by nature he was not at all mettlesome, he seemed to feel lively and began pawing—for as to capering, begging his pardon, he knew not what it meant.
But tell me, as thou livest, hast thou seen a more valiant knight than I in all the known world; hast thou read in history of any who has or had higher mettle in attack, more spirit in maintaining it, more dexterity in wounding or skill in overthrowing?
Don Quixote was standing by at the time, highly pleased to see his squire’s stoutness, both offensive and defensive, and from that time forth he reckoned him a man of mettle, and in his heart resolved to dub him a knight on the first opportunity that presented itself, feeling sure that the order of chivalry would be fittingly bestowed upon him.
…assaulted by only two men while they were so many, betook themselves to their stakes, and driving the two into the middle they began to lay on with great zeal and energy; in fact, at the second blow they brought Sancho to the ground, and Don Quixote fared the same way, all his skill and high mettle availing him nothing, and fate willed it that he should fall at the feet of Rocinante, who had not yet risen; whereby it may be seen how furiously stakes can pound in angry boorish hands.
"Ah, senor governor," said another, "what slackness of mettle this is!
There are no more uses of "mettle" in the book.
Show samples from other sources
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.