Concupiscence is the human inclination to prefer that which is sinful to that which is holy. We have this defect because of the fall of Adam and Eve.
The impulse to sin is not a sin in itself, and when we resist the impulse to sin, we build up the virtues.
Etymologically, “concupiscence” can refer to any intense form of human desire. Christian theology has given it a particular meaning: the movement of the sensitive appetite contrary to the operation of the human reason.
The apostle St. Paul identifies it with the rebellion of the “flesh” against the “spirit.” Concupiscence stems from the disobedience of the first sin. It unsettles human moral faculties and, without being in itself an offense, inclines individuals to commit sins.
Certain temporal consequences of sin remain in the baptized, such as suffering, illness, death, and such frailties inherent in life as weaknesses of character, and so on, as well as, an inclination to sin that Tradition calls concupiscence … since concupiscence is left for us to wrestle with, it cannot harm those who do not consent but heroically resist it by the grace of Jesus Christ. (8:47)