vouch

Synonyms: act as a witness affirm answer for assert asseverate assure attest to avert avow back bear testimony be responsible for certify confirm contend corroborate cosign declare get behind give an affidavit guarantee maintain okay predicate profess prove put forth rubberstamp say so sign for sponsor stand up for substantiate support swear to swear up and down testify uphold verify vow warrant witness

Antonyms: deny disavow refute reject renounce