The power of a spirit unyielding to any trial.

In the lower part of the "Author's Postface" (敘傳) of the Book of Han (漢書), compiled by the Later Han historian Ban Gu (32–92), appears the expression "不撓不屈" (buryobulgul). Ban Gu used it in describing his own family, and in particular the achievements of his elder brother Ban Chao (32–102). In the reign of Emperor Ming of the Later Han, Ban Chao was dispatched to the Western Regions with a retinue of only thirty-six men, and even in the extreme situation of facing down the Xiongnu (匈奴) he would not bend his purpose. He remained in the Western Regions for thirty-one years, drew more than fifty kingdoms into alliance, and reopened the Silk Road. Ban Gu compressed his brother's spirit into four characters: "不撓不屈" — he did not bend (撓) and was not broken (屈). The key here is the difference between "撓" and "屈." "撓" (yo) is to have one's direction twisted by an outside force, as bamboo bends in the wind, while "屈" (gul) is to be wholly broken and to submit, as one bends the knee. Buryobulgul, then, is a double negation: neither a change of direction (撓) nor a surrender (屈).

The English "fortitude" appeared in late-14th-century Middle English. By way of the Old French "fortitude," it comes from the Latin "fortitudo." Its root is "fortis" — the adjective "strong, sturdy, brave." From the same root come "fort" (fortress), "force" (strength), and "fortify" (to strengthen). In ancient Rome the word carried particular weight. Cicero (106–43 BCE), in his De Officiis, counted fortitudo among the four cardinal virtues (virtutes cardinales), together with justice (iustitia), temperance (temperantia), and wisdom (prudentia). The fortitudo Cicero spoke of was not the physical valor of the battlefield. He drew a clear distinction — "True fortitudo is not a reckless charge in the face of danger, but the greatness of soul (magnitudo animi) that endures pain and toil." Medieval Christianity adopted the virtue, and Thomas Aquinas (1225–1274), in his Summa Theologiae, redefined fortitudo as "the virtue that governs fear and restrains recklessness." According to the OED, the first English record is around 1400, in the sense of "moral courage and endurance."