"... let virtue garnish thy thoughts unceasingly." (D&C 121:45)
The root of the word "virtue" is simply "man." For example, virile is from the same root, as is werewolf, meaning man wolf. Happily, virtue has come to have much deeper meanings and senses over time.
The first definition of virtue in the Oxford English Dictionary is "the power or operative influence inherent in a supernatural or divine being." This is the meaning of the word in the Gospels of Mark and Luke, when a sick woman is healed immediately upon touching the Savior. "Jesus said, Somebody hath touched me: for I perceive that virtue is gone out of me" (Luke 8:46). In this same context, virtue also means "an act of superhuman or divine power; a mighty work; a miracle."
As Latter-day Saints, we rightly focus on the aspect of virtue that relates to chastity and sexual purity, which are also defined as manly virtues. Also, it is important to understand the larger sense of the word. The OED, in the sense relevant here, defines virtue as "conformity of life and conduct with the principles of morality; voluntary observance of recognized moral laws or standards of right conduct; abstention on moral grounds from any form of wrong-doing or vice."
The Apostle Peter teaches us that "according (to) his divine power (the Lord) hath given unto us all things that pertain unto life and godliness, through the knowledge of him that hath called us to glory and virtue: whereby are given unto us exceeding great and precious promises: that by these ye might be partakers of the divine nature, having escaped the corruption that is in the world through lust. (Therefore) giving all diligence, add to your faith virtue.... " (2 Peter 1:3-5)
It is no coincidence that Peter, having told us that we are called to glory and virtue to escape the lust of the world, teaches us that we need to add virtue to the foundational principle of faith. In D&C 121:45, we are taught that if we possess charity and virtue among other great promises, our "confidence will wax strong in the presence of God."
Confidence, of course, comes from the same root as faith. It means that we can trust or rely upon the Lord with assurance and certainty that we will receive his divine support.