From Longman Dictionary of Contemporary Englishavera‧ver /əˈvɜː $ əˈvɜːr/ verb (averred, averring) [transitive] formalTRUE to say something firmly and strongly because you are sure that it is true SYN declare→ See Verb table
Examples from the Corpus
aver• His mission, he would later aver, was wholly scientific.Origin aver (1300-1400) Old French averer, from Medieval Latin adverare “to show to be true”, from Latin ad- “to” + verus “true” 