Definition of back up phrasal verb from the Oxford Advanced American Dictionary

 

back up

 phrasal verb
phrasal verb
 
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Phrasal Verbs

    back up

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    back somethingup

     
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  1. 1to move backward, especially in a vehicle You can back up another two feet or so. I backed the car up to the door.
  2. 2to stop something from moving or flowing freely, usually by blocking it; to be unable to move or flow freely Traffic was backed up for about two miles because of the accident. Do your drains ever back up? related noun backup (3)

    back somebody/somethingup

     
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  1. 1to support someone or something; to say that what someone says, etc. is true I'll back you up if they don't believe you. The writer doesn't back up his opinions with examples.
  2. 2to provide support for someone or something two doctors backed up by a team of nurses The rebels backed up their demands with threats. related noun backup (1)

back somethingup

(computing)
 
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to prepare a second copy of a file, program, etc. that can be used if the main one fails or needs extra support It's a good idea to back up your files onto a memory stick. related noun backup (2)
See the Oxford Advanced Learner's Dictionary entry: back up