means The physical or mental endurance that lets you sustain prolonged effort or hardship without giving out.

from From Latin stamina, the plural of stamen, meaning the thread of life — literally the warp threads on a loom. In Roman myth the Fates spun these threads, and the length they spun determined how long you lasted; so 'stamina' began as the very stuff your life was made of. English borrowed it around the 18th century, first meaning the essential threads or constitution of a thing, then narrowing to the endurance those threads gave you. (And yes, it's the same word as the stamen in a flower, named for its threadlike shape.)