The term 'COMPLETELY REVERSE' can be summed up by the crossword solution 'UNDO, ' which is a four-letter word. To UNDO something means to reverse it, or to return it to its original state. This could mean reversing a decision, returning a situation to how it was before, or even taking apart something that has been put together. In any case, the term 'UNDO' implies a complete reversal of whatever has been done.