To mark a document with a date that is earlier than when it was actually written or created; to give something an earlier effective date.
From 'back' + 'date' (a calendar marking). This term emerged in the 1900s in legal and business contexts, literally meaning to put a date 'back' in time.
Backdating is technically illegal in many contexts—when executives backdate stock options, they're essentially creating a false historical record that can defraud shareholders and the government.
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