The story
From affy (“(obsolete) to have faith in, trust; to formally affirm or promise; etc.”) + -ant (suffix forming agent nouns from verbs).
Etymology from Wiktionary, CC BY-SA 4.0.
affiant
af·fi·antToday it means: An individual witness whose statement is contained in an affidavit; (generally) an individual who makes a sworn deposition; a deponent. Full definition →