Undo

google

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Old English undōn (see un-2, do1).

wiktionary

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From Middle English undōn, from Old English ondōn, from Proto-Germanic *andadōną(“to undo”), equivalent to un- +‎ do. Cognate with West Frisian ûndwaan, ûntdwaan(“to undo; rid”), Dutch ontdoen(“to undo”).

undo

etymonline

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undo (v.)

Old English undon "to unfasten and open" (a window or door), "to unfasten by releasing from a fixed position; to cancel, discharge, abrogate, reverse what has been done, put back in a former condition; bring to ruin, destroy," from un- (2) "opposite of" + do (v.). Related: Undone; undoing.