Onesimus (Ονησιμον). A common name among slaves and made like Chresimus, Chrestus. The word is from ονησις (profit) and that from ονινημ, to profit, to help.

Who was aforetime unprofitable to thee (τον ποτε σο αχρηστον). "The once to thee useless one." Play (pun) on the meaning of the name Onesimus (ονησιμος, useful) as once "useless" (αχρηστος, verbal adjective, α privative and χραομα, to use).

But now is profitable to thee and to me (νυν δε σο κα εμο ευχρηστον). "But now to thee and to me useful." Still further play on the name Onesimus by ευχρηστον (verbal adjective from ευ and χραομα, to use). Ethical dative here (σοι, εμο).