Swearing up and down is a phrase that means one is making an emphatic assertion or promise. Antonyms of this phrase include denying, negating, contradicting, and disavowing. When someone is denying something, they are stating that the opposite of what was said is true. Negating is the act of voiding or canceling something out. Contradicting involves making a statement that conflicts with what was previously stated. Lastly, disavowing means to refuse to acknowledge or accept something. All of these antonyms serve as alternatives to swearing up and down and can be used to express doubt or disbelief in someone's statements or actions.