affidavit (n.) Look up affidavit at Dictionary.com
"written declaration upon an oath," 1590s, from Medieval Latin affidavit, literally "he has stated on oath," third person singular perfective of affidare "to trust; to make an oath," from Latin ad- "to" (see ad-) + fidare "to trust," from fidus "faithful," from PIE root *bheidh- "to trust" (see faith). So called from being the first word of sworn statements.