An impurity is something that ruins the uncontaminated nature of something. If someone accuses you of impurity, they think you or your nature has been spoiled in some way by sin.
When water is pure, the only thing in that water is water. Add a contaminant, say salt or bleach, and you are introducing an impurity, something that turns the water impure. When used with people, the word has a religious overtone. Someone who is pure is unspoiled by sin. Think Snow White. If you convinced Snow White to rob a bank with you, you'd be spoiling her good nature with your wretched impurity.