JB Phillips wrote in his preface to Philemon that the slave's name Onesimus meant "useful" and it "explains the untranslatable pun in verse 11", where the text of the Phillips translation/ paraphrase says "Yes I have become a father though I have been under lock and key, and the child's name is -- Onesimus! Oh, I know you have found him useless in the past but he is going to be useful now, to both of us. I am sending him back to you..."
I don't think all commentators have agreed, but I would like to think Paul included that pun to lighten the mood and to bring a smile to his reader's face.