Meaning

noun written declaration made under oath; a written statement sworn to be true before someone legally authorized to administer an oath

originFrom 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).

Etymology from Wiktionary, CC BY-SA 3.0