1620s, "force irresistably, compel, oblige," also "make necessary, render unavoidable," from Medieval Latin necessitatus, past participle of necessitare "to render necessary," from Latin necessitas "compulsion; destiny" (see necessity). Earlier verb in English was necessen (late 14c.). Related: Necessitated; necessitates; necessitating.
c. 1200, vertu, "moral life and conduct; a particular moral excellence," from Anglo-French and Old French vertu "force, strength, vigor; moral strength; qualities, abilities" (10c. in Old French), from Latin virtutem (nominative virtus) "moral strength, high character, goodness; manliness; valor, bravery, courage (in war); excellence, worth," from vir "man" (from PIE root *wi-ro- "man").
For my part I honour with the name of virtue the habit of acting in a way troublesome to oneself and useful to others. [Stendhal "de l'Amour," 1822]
Especially (in women) "chastity, sexual purity" from 1590s. Phrase by virtue of (early 13c.) preserves alternative Middle English sense of "efficacy." Wyclif Bible has virtue where KJV uses power. The seven cardinal virtues (early 14c.) were divided into the natural (justice, prudence, temperance, fortitude) and the theological (hope, faith, charity). To make a virtue of a necessity (late 14c.) translates Latin facere de necessitate virtutem [Jerome].