The Greek verb aneko (ἀνήκω) means to be fitting, proper, or appropriate — to belong to the category of things that are right or suitable in a given context. It appears three times in the New Testament (Ephesians 5:4; Colossians 3:18; Philemon 8), always in ethical contexts.
Aneko carries the nuance of social and moral fittingness — what belongs in a certain sphere, what is becoming for a person in their relationship to God and others. Paul uses it to rule out obscenity and foolish talk (Ephesians 5:4), to describe the proper relationship of wives to husbands "as is fitting in the Lord" (Colossians 3:18), and to justify his appeal to Philemon on behalf of Onesimus as "the fitting thing" (Philemon 8). The concept is not about rigid rules but about actions that belong in the kingdom of God — that cohere with who we are in Christ. Grace transforms not just actions but the whole sense of what is appropriate for a new creation person.