Catholic Catechism:
2515 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.