unhappen means to cease to have happened; to undo itself. Lexicurio rates it Sui generis — a strength score of 89 out of 100.
Why “unhappen” is a great word
To cease to have happened; to undo or reverse the occurrence of an event. Formed within English by derivation, from the prefix un- (expressing reversal) + the verb happen. Unlike "erase," which scrapes away a physical record, or "prevent," which forestalls a future possibility, "unhappen" operates in the impossible, metaphysical past. It is the longing to dissolve a spoken word back into silence, to feel the heat of a spilled cup flow upward into its vessel, to watch a shadow retreat up a sundial—a quiet, human admission that some stains cannot be laundered, only wished away.
Etymology
From un- + happen.
verb
- To cease to have happened; to undo itself.“I wish everything that happened between us could unhappen, and we could start again.”