An amnesty, at the root, is a deliberate act of collective memory loss.
Etymonline traces the word to Greek amnestia, "forgetfulness, especially of wrongs." It joins the negative a- to mnestis, "remembrance," which is related to mnaomai, "I remember." The adjective amnestos meant "forgotten" or "forgetful." The word came into English in the 1570s, through Latin and the French amnistie. There it meant a pardon of past offenses by a ruling power, usually for a whole class of crimes against the government.
The idea has an ancient model. The 1911 Encyclopaedia Britannica calls the amnesty of Thrasybulus at Athens the earliest one on record. Thrasybulus led the democratic resistance to the Thirty Tyrants, and democracy was restored in 403 BC. An Oxford TORCH article explains the deal: no one was to remember past wrongs. The period of the Thirty could not be raised in the law courts. The Thirty themselves, and a few others, were left out.
That exclusion became a pattern. When Charles II returned to the English throne, his amnesty did not cover the men who had condemned his father, Charles I, to death. An older political encyclopedia, hosted by Econlib, makes the philosophical point plainly. To grant an amnesty "is not to forgive or pardon; it is to forget."
The word's closest relative makes the same point medically. Amnesia, "loss of memory," arrived in English in 1786 from the closely related Greek amnesia. Amnesty is the version a government chooses. Amnesia is the version that happens to you.
Both belong to a large family built on the Indo-European root men-, "to think." Etymonline links it to mnemonic, memento, mention and mind. It even lists monument, a structure meant to make people remember. Amnesty is the one member of the group whose job is the opposite.
English did not use amnesty as a verb until 1809. The noun is far older, and its promise has stayed the same: the past will not be held against you.