[1] : 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

[2] : The Collaborative International Dictionary of English v.0.48

back up \back" up\, v. t.
   1. to serve as a backup3 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 backup5 of; as, the sysop backed
      up the purchasing data files every night.
      pjc

[3] : WordNet (r) 2.0

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
     2: move backwards from a certain position; "the bully had to
        back down" syn: back off, back down
     3: establish as valid or genuine; "can you back up your
        claims?" syn: back
     4: make a copy of a computer file especially for storage in
        another place as a security cop; "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, congest, choke, foul ant:
         unclog
see also:
support back off back down back clog choke off 
clog up congest choke foul unclog 

[4] : 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

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