The Greek verb porneuo (πορνεύω) means to commit sexual immorality, fornication, or prostitution. It appears about 8 times in the NT. The related noun porneia (G4204) is more common. Both derive from porne (prostitute).
Porneuo describes sexual sin outside of the covenant of marriage. In Revelation, it takes on cosmic proportions: the great prostitute who corrupted the earth by her adulteries (Revelation 19:2) represents spiritual adultery against God through idolatry. Paul connects sexual immorality to idolatry repeatedly (1 Corinthians 10:8). The body matters: 'Flee from sexual immorality. All other sins a person commits are outside the body, but whoever sins sexually, sins against their own body' (1 Corinthians 6:18). The call to flee porneia is not legalism — it is covenant faithfulness, because the body belongs to the Lord and is a temple of the Holy Spirit.