In 1888, Nietzsche wrote “Aus der Kriegsschule des Lebens.—Was mich nicht umbringt, macht mich stärker,” which can be translated as “Out of life’s school of war—what doesn’t kill me, makes me stronger.” It appears in his book of aphorisms, Twilight of the Idols, and no further explanation follows. Nietzsche expanded on the idea somewhat in his autobiography Ecce Homo, also composed in 1888. Here, he refers to select individuals as “nature’s lucky strokes…among men,” and says of such a person, “He divines remedies for injuries; he knows how to turn serious accidents to his own advantage; that which does not kill him makes him stronger.” University of Glasgow philosophy professor Michael Brady explains that Nietzsche does not seem to think that all suffering will result in strength, but rather that he is suggesting one should take suffering as an opportunity to build strength, and that those who are already strong are those who can do so.

Source: Dictionary.com