iniquity: sin pictured as a crooked thing that clings to the person who did it, carrying within it the deed, the guilt it creates, and the ruin it brings
This is one of the Hebrew Bible's heaviest words for sin, and the reason it is hard to pin down is that it does three jobs at once. It can name the crooked act itself, the guilt that act fastens onto the wrongdoer, and the disaster that eventually comes back on them. Older readers thought these were three separate meanings; most scholars now see them as three angles on a single idea: sin is not just an event that happened and is over, it is a thing with weight that sticks to a person and travels with them.
The underlying picture is bentness. The root behind the word means something like to bend, twist, or go off the straight path, so the word does not point first to a single wrong action but to a condition of crookedness in the person and their dealings. That is why the same word can slide from the wrong itself, to the liability it leaves, to the punishment that closes the circle. In Genesis 4:13 Cain cries that his iniquity is greater than he can bear, and translators have to choose one word while the verse means all three together.
The most telling thing the Bible does with this word is to speak of bearing it, lifting it, carrying it off. A guilty person bears their own and is crushed; but a priest, the live goat on the Day of Atonement, or God can bear it for them, and then the load is gone. The senses below trace the three angles, but they belong to one connected reality, which is why a single word holds them.