There is an allusion to the meaning of the name Onesimus’ (Greek Ὀνήσιμος), “profitable or useful,” in v. 11 by means of the synonym εὔχρηστος (“profitable”) and its antonym ἄχρηστος (“unprofitable”). However, the actual pun occurs in v. 20:
Κʹ ναὶ ἀδελφέ, ἐγώ σου ὀναίμην ἐν κυρίῳ· ἀνάπαυσόν μου τὰ σπλάγχνα ἐν Χριστῷ. NA28, ©2012
20 Yes, brother, let me benefit from you in the Lord; refresh my heart in Christ. NASB, ©1995
Heinrich August Wilhelm Meyer commented on Philemon 1:20,
According to Wilke (translated by Thayer),3 Ὀνήσιμος is derived from the noun ὄνησις (“profit”), which is related to the verb ὀνίνημι (“to profit”).4 The word ὀναίμην in v. 20 is a verb conjugated from ὀνίνημι.
1 Meyer (translated by Moore), p. 413
2 Meyer, p. 537
3 Wilke, p. 447
4 LSJ, p. 1231
Liddell, Henry George; Scott, Robert; et al. A Greek-English Lexicon. 9th ed. with revised supplement. Oxford: Clarendon, 1996.
Meyer, Heinrich August Wilhelm. Critical and Exegetical Handbook to the Epistles to the Philippians and Colossians, and to Philemon. Trans. Moore, John C. Ed. Dickson, William P. New York: Funk, 1889.
Meyer, Heinrich August Wilhelm. Kritisch exegetischer Kommentar über das Neue Testament, Neunte Abtheilung, Kritisch exegetisches Handbuch über die Briefe Pauli an die Philipper, Kolosser und an Philemon. 5th ed. Vol. 9. Göttingen: Vandenboeck and Ruprecht, 1886.
Wilke, Christian Gottlob. A Greek-English Lexicon of the New Testament: Being Grimm Wilke’s Clavis Novi Testamenti. Trans. Thayer, Joseph Henry. Ed. Grimm, Carl Ludwig Wilibald. Rev. ed. New York: American Book, 1889.