Seven Deadly Sins: Lust

1.
Lust: From the Anglo-Saxon lust, 'pleasure'; the Old Norse losti, 'sexual desire'; the Medieval Latin lasciviosus, 'wanton', 'lustful'; the Latin lascivus, 'wanton', originally 'playful' as applied to children and animals; the Greek laste, 'a wanton woman', lasthe, 'a mockery', and lilaiesthai, 'to yearn'; and the Sanskrit lasati, 'he plays', and lalasas, 'desirous'. There is no reference to lust in the four Gospels. However, the terms orezis, 'appetite', epithumetas, 'desire of the heart', and hedone, 'pleasure', occur about two dozen times in the Epistles, almost always in a negative context.

2.
”To crave the pleasure of the body.”
Lust in the modern sense is described as a powerful sexual desire, but it wasn't always so. The Latin word used by early Church leaders was luxuria, and the transgression it described wasn't specific to sexuality but to sensuality. Luxuria was an inordinate craving for the pleasures of the body, and it was a sin because of its dulling effect on the spiritual senses.
Later, the focus would shift to sexual lust by medieval theologian Thomas Aquinas, whose 13th-century blockbuster Summa Theologica described the sin in more detail. Lust was the culprit when people engaged in masturbation, same-sex coupling, bestiality and "not observing the natural manner of copulation."
One good thing about lust was that because it was a sin of the flesh, it couldn't persist into eternity; this is at least partly why it was categorized as the least egregious of the Seven Deadly Sins. In Dante's Inferno, the lustful were sent to the second circle of hell, where they were forever blown about as if in a great tornado, reflecting the stormy passions that gripped them on Earth. (Compare this to the punishment for flatterers, who were relegated to the eighth circle and punished with an eternal covering of filth.)
The modern Church's view is that we fall victim to lust when we express our sexuality outside the context of marriage. Lust can even exist within marriage if either partner views the other as a sex object rather than a human being.