A variant spelling of 'affidavit,' a written statement made under oath or solemn promise; a legal declaration of facts.
From Medieval Latin 'affidavit' (he has pledged), related to 'affy' and combining Latin elements. 'Affydavy' is a dialectal or archaic variant of the more common 'affidavit' spelling.
The spelling 'affydavy' shows how legal terminology sounded to ordinary people trying to spell it phonetically—it's basically what someone heard in court and wrote down, revealing the gap between Latin legal language and common speech.
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