Verses 19-21 are warnings against committing adultery. The reason given is that it is unwise. A man who commits adultery with another man’s wife will have children who grow up in another man’s house. Compare Pro 5.15-17, where the idea is expressed, as it is here, in metaphors.

My son, keep sound the bloom of your youth, and do not give your strength to strangers: My son is an address to the readers as the writer’s students. See the comments on 2.1. The expressions the bloom of your youth (literally “the peak of your age/manhood/strength”; compare GrkEst 15.5, where Esther appears “at the peak of her beauty” [Revised Standard Version “with perfect beauty”]) and your strength refer to having children. See Gen 49.3, where Jacob refers to Reuben as “my might” and “the first fruits of my strength”; the Hebrew nouns for “might” and “strength” used there refer to “virility, manhood.” It cannot be accidental that the Greek terms used here for youth and strength are perfect translations of two Hebrew terms in that verse. (There is no Greek word in this verse that means bloom and no Greek word that specifically means youth.) An alternative model for the first line of this verse is “My son [or, Students] stay healthy while you are still virile [or, at the peak of your manhood].” Strangers here is usually taken to refer to women (although the Greek word is masculine), any woman other than one’s wife, but it surely refers to men. The idea is that a young man should not give his children to a stranger (another man) by committing adultery with another man’s wife (any child born would grow up as the other man’s child). See Pro 5.15-17; compare Pro 5.10. Translators have a choice of translating this verse fairly literally and keeping the metaphors, or rewording it in order to express the meaning of the metaphors (as Good News Translation did at Pro 5.15). We recommend the latter option. For the last line of this verse we could say “and do not allow [or, make it possible for] some stranger to call your child his own.”

Quoted with permission from Bullard, Roger A. and Hatton, Howard A. A Handbook on Sirach. (UBS Helps for Translators). New York: UBS, 2008. For this and other handbooks for translators see here.