Adam Clarke on Matthew 5:33

Thou shalt not forswear thyself - They dishonor the great God, and break this commandment, who use frequent oaths and imprecations, even in reference to things that are true; and those who make vows and promises, which they either cannot perform, or do not design to fulfill, are not less criminal. Swearing in civil matters is become so frequent, that the dread and obligation of an oath are utterly lost in it. In certain places, where oaths are frequently administered, people have been known to kiss their thumb or pen, instead of the book, thinking thereby to avoid the sin of perjury; but this is a shocking imposition on their own souls. See the notes on Deu 4:26; Deu 6:13.

Perform unto the Lord thine oaths - The morality of the Jews on this point was truly execrable: they maintained, that a man might swear with his lips, and annul it in the same moment in his heart. Rab. Akiba is quoted as an example of this kind of swearing. See Schoettgen.