Affidavit

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mid 16th century: from medieval Latin, literally ‘he has stated on oath’, from affidare .

wiktionary

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From Medieval Latin affidavit(“he has sworn”), the third person singular perfect tense of affido(“swear”), from fīdō, ultimately from Proto-Indo-European *bʰeydʰ-(“to command, to persuade, to trust”). Cognate to fidelity and faith (same Latin root), but not to affirm (shared Latin ad- prefix, but different Latin and Proto-Indo-European roots).

etymonline

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affidavit (n.)

"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, confide, persuade." So called from being the first word of sworn statements.