What is concupiscence?
In its widest acceptation, concupiscence is any yearning of the soul for good; in its strict and specific acceptation, a desire of the lower appetite contrary to reason.
The Catechism of the Catholic Church (2515) puts it this way:
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 man’s moral faculties and, without being in itself an offense, inclines man to commit sins.
Catholic author and psychotherapist Gregory K. Popcak has developed a useful metaphor for envisioning concupiscence: Think of a garden hose that you left lying unravelled in your driveway. Before you bothered to wind it up and store it correctly you ran over it a few times with your car as you entered and exited the driveway. Finally, you washed the hose off and were ready to wind it up. While the hose is clean (baptism), you are going to have a difficult time winding it. The hose will “remember” its previous state and “fight” being coiled neatly (concupiscence).
As the Catechism notes, concupiscence is not a sin in and of itself, but it is that part of us that fights against right reason and inclines us toward sin.