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]

Moby Thesaurus II by Grady Ward, 1.0:

144 Moby Thesaurus words for "back up":
   act for, advance, affirm, afford support, answer for, appear for,
   ascend, attest, authenticate, back, back away, back off, backpedal,
   backtrack, backtrail, backwater, bear, bear out, bear up, bolster,
   bolster up, brace, budge, buoy up, buttress, carry, certify,
   champion, change, change place, circle, circumstantiate, climb,
   come after, come last, commission, confirm, corroborate,
   countermarch, cradle, crutch, cushion, deputize, descend, document,
   ebb, elect, endorse, fall astern, fall back, fall behind, flow,
   follow, fortify, front for, get behind, get in behind, get over,
   give support, go, go around, go back, go backwards,
   go into reverse, go round, go sideways, gyrate, hold, hold up,
   keep afloat, keep up, lag behind, lend support, mainstay, maintain,
   make sternway, mount, move, move over, nominate, pillow,
   pinch-hit for, plunge, probate, progress, prop, prove, ratify,
   regress, reinforce, represent, retrogress, reverse, revert, rise,
   rotate, run, run interference for, second, shift, shore, shore up,
   shoulder, side with, sink, soar, speak for, spin, stand back of,
   stand behind, stand by, stand in for, stay, stick by, stick up for,
   stir, stream, strengthen, subside, subsidize, substantiate,
   substitute for, subvention, support, sustain, take sides with,
   trail, trail behind, travel, underbrace, undergird, underlie,
   underpin, underset, understudy, upbear, uphold, upkeep, validate,
   verify, vote, wane, warrant, whirl