Perfect passive participle of exerceō (“to occupy oneself, oversee; work at, practice, exercise”), from ex- (“out of”) + arceō (“to ward off, protect, guard”), from Proto-Italic *arkeō, from Proto-Indo-European *h₂orké-ye-ti, from Proto-Indo-European *h₂erk- (“to protect, guard”).
Practice speaking and memorizing "exercitus" and many other words and sentences in Latin.