perjure oneself

The Fox's face fell at these words, for although he was both untruthful and dishonest now, he had been well brought up in his youth, and he knew that it was a terrible thing to perjure oneself and swear falsely.

To perjure oneself, according to the law, you have to make a statement that is contrary to what you believe to be true.

For the line "I know that what I'm going to do is bad,"666 is especially applicable to people that importune us, when one is going to perjure oneself, or deliver an unjust verdict, or vote for a measure that is inexpedient, or borrow money for someone who will never pay it back. § xii.

To manswear comes from the Anglo-Saxon mánswerian meaning to swear falsely or to perjure oneself.

"Neither do I," she returned, "any more than I do in the kind of honor that compels a man to pay a gambling debt before he pays his tailor, but I do believe that there may be situations where, though it would not be permissible to perjure oneself, honor would require one to refuse to obey the law."