undoundo /ʌnˈdu/ verb (past tense undid /ʌnˈdɪd/, past participle undone /ʌnˈdʌn/, undoes /ʌnˈdʌz/) 1 
to open something that is fastened► undo
to open something that is fastened► undo to open something that is fastened, tied, or wrapped: I was trying to undo the knot in the string. He took off his tie and undid the top button of his shirt.
► unfasten to open something that is fastened or tied: Lewis unfastened his seat belt and got out of the car.
► untie to undo string, rope, etc. that someone has tied in a knot: I untied the rope and got into the boat.
► unscrew to remove something by twisting it or taking screws out: Turn off the light before unscrewing the bulb.
► unzip to unfasten clothing, a bag, etc. by opening the zipper on it: Lucy unzipped her jacket because she was hot.
► unbutton to unfasten a piece of clothing that is closed with buttons: He unbuttoned his shirt and took it off.
2 to change something back to how it was before► undo to change something back to how it was before: One careless mistake could undo all our good work and spoil everything. It is going to be difficult to undo the effects of years of discrimination.
► reverse AWL to change something so that it goes back to what it was before, or so that it is the opposite: It will take years to reverse the damage done by pollution. The Supreme Court reversed the lower court's decision.
- become extinct
- become familiar with
- become quiet
- bed
- bed and breakfast
- be defeated
- be deficient in
- be devoted to
- be divided
- be employed
- be enclosed by
- be engrossed in something
- be entitled
- be entitled to
- beep
- be expected to
- be falling apart
- be fired
- be five feet/six feet etc. tall
- be fluent in
- before
- beforehand
- before long
- be found
- be founded on