He that falls obstinate in his courage — Si succiderit, de genu pugnat — he who, for any danger of imminent death, abates nothing of his assurance; who, dying, yet darts at his enemy a fierce and disdainful look, is overcome not by us, but by fortune; he is killed, not conquered.
The most valiant are sometimes the most unfortunate.
There are defeats more triumphant than victories.

[Celuy qui tombe obstiné en son courage, si succiderit, de genu pugnat. Qui pour quelque danger de la mort voisine, ne relasche aucun point de son asseurance, qui regarde encores en rendant l’ame, son ennemy d’une veuë ferme & desdaigneuse, il est battu, non pas de nous, mais de la fortune: il est tué, non pas vaincu: les plus vaillans sont par fois les plus infortunez. Aussi y a-il des pertes triomphantes à l’envy des victoires.]

Michel de Montaigne (1533-1592) French essayist

Essays, Book 1, ch. 30 (1.30), “Of Cannibals [Des Cannibales]” (1578) [tr. Cotton/Hazlitt (1877)]
    (Source)

The Latin phrase is from Seneca, De Provdentia [On Providence], 1.2. It means "If his legs fail him he fights on his knees."

Note this was inserted into this passage only in the final, 1595, edition, as was the final sentence (defeats greater than victories). The most-valiant/most-unfortunate sentence was an addition in the 1588 edition.

As examples of the concluding sentence, he goes on to compare great victories (Salamis, Plataea, Mycale, Sicily) to the "defeat" of Leonidas and his Spartans at Thermopylae.

Some editions use the 1588 sequence of chapters, not the 1595, and so identify this as ch. 31.

(Source (French)). Alternate translations:

Hee that obstinately faileth in his courage, Si succiderit, de genu pugnat. He that in danger of imminent death, is no whit danted in his assurednesse; he that in yeelding up his ghost beholdeth his enemie with a scornefull and fierce looke, he is vanquished, not by us, but by fortune: he is slaine, but not conquered. The most valiant, are often the most unfortunate. So are there triumphant losses in envie of victories.
[tr. Florio (1603)]

He that falls obstinate in his courage -- Si succiderit, de genu pugnat; -- he who, for any danger of imminent death, abates nothing of his assurance; who, dying, yet darts at his enemy a fierce and disdainful look, is overcome not by us, but by fortune; he is killed, not conquered; the most valiant are sometimes the most unfortunate. There are defeats more triumphant than victories.
[tr. Cotton (1686)]

The man who falls obstinately courageous, si succiderit, de genu pugnat. He who does not flinch, be he in ever such imminent danger of death, and who, when giving up the ghost, looks his enemy in the face with a stern and disdainful countenance, he is conquered not by us, but by fortune; nay, he is killed, not conquered; the most valiant being sometimes the most unfortunate. There are actually some defeats which may compare even with victories for triumph.
[tr. Friswell (1868)]

He who falls persistent in his will, si succiderit, de genu pugnat. He who abates no whit of his firmness and confidence for any danger form death not far away; he who, while yielding up his soul, still gazes at his foe with an unshrinking and disdainful eye -- he is beaten, not by us, but by fortune; he is killed, not conquered. The most valiant are sometimes the most unfortunate. So too there are defeats no less triumphant than victories.
[tr. Ives (1925)]

He who falls obstinate in his courage, if he has fallen, he fights on his knees [Seneca]. He who relaxes none of his assurance, no matter how great the danger of imminent death; who, giving up his soul, still looks firmly and scornfully at his enemy -- he is beaten not by us, but by fortune; he is killed, not conquered.
The most valiant are sometimes the most unfortunate. Thus there are triumphant defeats that rival victories.
[tr. Frame (1943), 1.31]

He who falls with a firm courage, "will, though fallen, fight on his knees." The man who yields no jot to his steadfastness for any threat of imminent death, who, as he yields up his soul, still gazes on his enemy with a firm and disdainful eye, is beaten not by us but by fortune; he is killed but he is not vanquished. The most valiant are sometimes the most unfortunate.
There are defeats, therefore, that are as splendid as victories.
[tr. Cohen (1958)]

The man who is struck down but whose mind remains steadfast, "si succiderit, de genu pugnat," the man who relaxes none of his mental assurance when threatened with imminent death and who faces his enemy with inflexible scorn as he gives up the ghost is beaten by Fortune not by us: he is slain but not vanquished. Sometimes it is the bravest who may prove most unlucky. So there are triumphant defeats rivalling victories.
[tr. Screech (1987), 1.31]

The man who falls, persevering in his courage, si succiderit, de genu pugnat. A man who does not relax any of his assurance despite the imminence of death -- who still gazes firmly and disdainfully at his enemy as he gives up the ghost -- is defeated not by us but by fortune'; he has been slain, not vanquished. Sometimes the most valiant are the most ill-fortuned. Thus there are triumphant defeats, rivaling victories.
[tr. Atkinson/Sices (2012)]