The Collaborative International Dictionary of English v.0.48:

back up \back" up\, v. t.
   1. to serve as a backup[3] for (another person or persons);
      as, the patrolmen backed up the detectives as they went
      inside to make the arrest; the center fielder backed up
      the shortstop on the play.
      [PJC]

   2. (Computers) to make a backup[5] of; as, the sysop backed
      up the purchasing data files every night.
      [PJC]

The Collaborative International Dictionary of English v.0.48:

back up \back" up\, v. i.
   1. to move in a reverse direction; -- used of vehicles or
      animals.
      [PJC]

   2. to accumulate due to a blockage of flow; as, a traffic
      backup due to an accident; a sewage backup.
      [PJC]

WordNet (r) 3.0 (2006):

back up
    v 1: give moral or psychological support, aid, or courage to;
         "She supported him during the illness"; "Her children
         always backed her up" [syn: support, back up]
    2: move backwards from a certain position; "The bully had to
       back down" [syn: back up, back off, back down]
    3: establish as valid or genuine; "Can you back up your claims?"
       [syn: back, back up]
    4: make a copy of (a computer file) especially for storage in
       another place as a security copy; "You'd better back up these
       files!"
    5: become or cause to become obstructed; "The leaves clog our
       drains in the Fall"; "The water pipe is backed up" [syn:
       clog, choke off, clog up, back up, congest,
       choke, foul] [ant: unclog]