1.  Move or drive a vehicle backward, as in He told her to back up into the garage. [First half of 1800s]
2.  Bring or come to a standstill, as in The water had backed up in the drains, or The accident had backed up traffic for miles. [First half of 1800s]
3.  Support or strengthen, as in The photos were backed up with heavy cardboard so they couldn't be bent, or I'll back up that statement of yours. [Second half of 1700s]
4.  Duplicate a file or program so that the original is not lost. For example, Every computer manual warns you to back up your work frequently in case of a power outage or computer failure. [Second half of 1900s]

The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Company. All rights reserved.