The commonly used expression "swore up and down" means to make a solemn declaration or statement, usually in a vehement manner, that something is true or false. Antonyms of this phrase include expressions like "deny vehemently," "contradict," or "oppose." These phrases suggest a refusal to agree or accept someone else's statement or explanation. Other antonyms for "swore up and down" may include "doubt," "disbelieve," "suspect," "question," and "challenge." Each of these antonyms implies a lack of trust or confidence in the truthfulness of a statement or belief. Overall, antonyms of "swore up and down" provide alternative ways to express the opposite of making a firm declaration or statement.