C. F. Keil and Franz Delitzsch on Leviticus 5:4

Lev 5:4

The third was, if any one should “swear to prate with the lips,” i.e., swear in idle, empty words of the lips, - “to do good or evil,” i.e., that he would do anything whatever (Num 24:13; Isa 41:23), - “with regard to all that he speaks idly with an oath,” i.e., if it related to something which a man had affirmed with an oath in thoughtless conversation, - “and it is hidden from him,” i.e., if he did not reflect that he might commit sin by such thoughtless swearing, and if he perceived it afterwards and discovered his sin, and had incurred guilt with regard to one of the things which he had thoughtlessly sworn.