A man who swears many oaths will be filled with iniquity: Good News Translation “sinful to the core” is an English idiom for filled with iniquity. Alternative renderings for this line are “Anyone who is always taking oaths is thoroughly sinful” and “Anyone who continually makes promises using the name of the Lord is a completely sinful person.”
And the scourge will not leave his house: The scourge, literally a whip or lash, symbolizes punishment. His house refers to the person who is always taking oaths, his family, and all his belongings. It is difficult to express his house in English, but for the whole line we could say “and he and his family will be punished constantly” or “and he and his family will never be free of punishment.” In languages that do not have the passive voice, we may say “and the Lord will punish him and his family continually.”
If he offends, his sin remains on him: Offends refers to taking an oath and not fulfilling it. For the whole line Good News Translation has “If he fails to fulfill his oath, he is guilty,” or we may say “If he fails to do what he promises….”
And if he disregards it, he sins doubly: Disregards refers to ignoring what he has sworn to do. Good News Translation renders this line as “If he ignores his oath, he is twice as guilty.” We may also say “If he forgets his promise on purpose, this makes him twice guilty.”
If he has sworn needlessly, he will not be justified: If he has sworn needlessly means “if the oath was really not necessary, if it was made casually or lightly.” Good News Translation “insincere” shifts the focus a bit, but not badly; “unnecessary” might be a bit better. He will not be justified means “he cannot possibly be forgiven.” So for this whole line we may translate “If he has made his promise unnecessarily [or, insincerely], he cannot possibly be forgiven,” or even “… the Lord will not forgive him.”
For his house will be filled with calamities: Good News Translation “house full of trouble” is almost literal, but happens to work well in English. His house means the same here as in the second line of the verse. For this line we could say “As a result, his life and the life of his family will be full of [or, plagued with] troubles.”
An alternative model for this verse is:
• Anyone who continually makes promises [or, takes oaths] using the name of the Lord is a completely sinful person and the Lord will punish him and his family continually. If he fails to do what he promises, he is guilty of sin. If he ignores his promise on purpose, this makes him twice guilty. If he made his promise unnecessarily [or, insincerely], the Lord will not forgive him. As a result, his life and the life of his family will be full of troubles.
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.