it is fitting: what is proper, becoming, or one's plain duty to do
In the New Testament this word is used impersonally to say that something "is fitting" or "proper." It names what belongs to a situation, what a person owes, or what one ought to do. It began as a concrete word for physically reaching or coming up to a level, then grew to mean "to belong to" or "to be due to" someone, and by Paul's day it had become an ordinary way of saying "it is proper."
Paul uses it three times, always to mark a standard of conduct. Coarse and shameful talk is "not fitting" for God's people (Ephesians 5:4); a wife's devotion to her husband is "fitting in the Lord" (Colossians 3:18); and Paul says he could simply command Philemon to do "what is fitting," his plain duty, but chooses to appeal to love instead (Philemon 1:8).
The language of "what is fitting" was ordinary moral vocabulary in the Greek-speaking world, close to the philosophers' standard term for proper action (καθῆκον). Paul's contribution is not the word but its new anchor: he takes this shared cultural language and grounds it "in the Lord," so that what is proper is measured by Christ rather than by custom or reason alone.