Achreios (ἀχρεῖος) combines a (not) and chreios (useful). It means "worthless, unprofitable, unserviceable." It appears twice: Matthew 25:30 (the wicked servant cast out) and Luke 17:10 (faithful servants calling themselves "unworthy servants").
The two NT uses create profound tension. In Matthew 25:30, the servant who buried his talent is called achreios — worthless, cast out. In Luke 17:10, faithful servants who did everything commanded call themselves achreios — unworthy. The first warns against unfaithful passivity; the second models humble service without entitlement. Even maximally faithful Christians owe God everything and can claim no merit. Salvation and reward flow entirely from God's grace — not from achieving "useful" status (Romans 3:12).