The term 'undo legally' can be summarized by the crossword solution 'ANNUL'. Annulment is a legal process that allows a marriage to be declared void, as if it never existed. It is different from divorce in that it does not require a court hearing or a waiting period. Annulment is typically used when a marriage was entered into under false pretenses or when one of the parties was underage or mentally incapacitated at the time of the marriage.