1) To move backwards, either on foot or in a vehicle.
2) To give intellectual or logistical support to another:
Pupok backed Enkidu up by noting that she too had seen the aliens, and demonstrated her many impressive scars.
or
While Pupok rushed the pillbox with explosives, Enkidu provided backup with his gat.
3) To make a copy of data, usually data stored in electronic format, in case the copy in regular use is damaged. Also applied to the copy itself, though usually shortened to backup.
4) To move backwards metaphorically, to make one's position in an argument less extreme.

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