early 13c., "moral life and conduct, moral excellence," vertu, from Anglo-French and Old French vertu, from Latin virtutem (nominative virtus) "moral strength, manliness, valor, excellence, worth," from vir "man" (see virile).
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].