"To write an affidavit" means to draft a formal written statement, usually witnessed and signed under oath, that provides detailed information or evidence about a specific event, situation, or issue. Full definition
I read an affidavit of someone who wrote an affidavit for the other side. (law.stackexchange.com)
I guarantee that the opposing party is not going to write an affidavit admitting to everything. (slaw.ca)
What are the options if a fact witness in a civil case writes an affidavit that is filed with the court and later finds out that the affidavit is simply wrong? (law.stackexchange.com)