shameless excess: unrestrained conduct that openly flouts common decency, sin done in the open rather than hidden
The constant in this word is not what the sin is but how it is done: out in the open, with no shame and no attempt to hide. A person can do wrong quietly and cover it up, but this word names wrongdoing that has stopped caring who sees, the kind that dares anyone to object. What that shameless conduct actually consists of shifts from passage to passage.
At one end it is brazen moral abandon in general. In the list of things that defile a person from within (Mark 7:22), and in the picture of people who have given themselves over to every kind of impurity (ἀκαθαρσία) along with greed (Ephesians 4:19), the word is broader than sex: it is a whole posture of unashamed defiance. At the other end it is open sexual license, as in the nighttime revelry Paul warns against (Romans 13:13) and the teachers in Jude and 2 Peter who treat God's grace as permission to do whatever they please.
Because English has no single word that carries both the shamelessness and the range of content, translators reach for several: "licentiousness," "debauchery," "sensuality," "lewdness," "wantonness." Each captures part of it and misses the sharpest note, which is public, unblushing defiance. Scholars still debate how much the New Testament writers narrowed the word toward sexual sin specifically, and how much they kept its older, wider force.