Akathartos (G169) is the negative of katharos (clean, pure). In the NT it appears frequently in the phrase pneuma akatharton ('unclean spirit') — the demonic. It also describes moral impurity in ethical lists. The term carries both ritual and moral connotations.
The frequent use of akathartos for demonic spirits reveals the holiness-dimension of Jesus' ministry. When he casts out unclean spirits, he is doing exactly what the OT purity code pointed toward: restoring cleanness, driving out what defiles. The ritual uncleanness of Leviticus finds its deepest fulfillment in Christ's authority over spiritual impurity. The gospel declares believers 'washed,' 'sanctified,' 'made clean' through Christ (1 Corinthians 6:11).