WordNet (r) 3.0 (2006):

keep back
    v 1: keep under control; keep in check; "suppress a smile";
         "Keep your temper"; "keep your cool" [syn: restrain,
         keep, keep back, hold back]
    2: secure and keep for possible future use or application; "The
       landlord retained the security deposit"; "I reserve the right
       to disagree" [syn: retain, hold, keep back, hold
       back]
    3: hold back; refuse to hand over or share; "The father is
       withholding the allowance until the son cleans his room"
       [syn: withhold, keep back]

Moby Thesaurus II by Grady Ward, 1.0:

149 Moby Thesaurus words for "keep back":
   abstain, arrest, backpedal, backwater, block, bosom, bottle up,
   brake, bridle, check, classify, clip the wings, conceal, confine,
   conserve, constrain, contain, control, cool, cool off,
   countercheck, curb, curtail, dam up, damp, dampen, decelerate,
   delay, detain, disallow, dispense with, do without, dompt, drag,
   draw rein, ease off, ease up, enjoin, file and forget, forbear,
   forgo, govern, guard, hinder, hold, hold at bay, hold back,
   hold fast, hold in, hold in check, hold in leash, hold off,
   hold out on, hold up, husband, impede, in petto, inhibit,
   intercept, interfere, intermeddle, interrupt, intervene, keep,
   keep between us, keep buttoned up, keep by one, keep close,
   keep dark, keep from, keep in, keep in check, keep in hand,
   keep in ignorance, keep in reserve, keep in store, keep mum,
   keep on hand, keep out, keep secret, keep snug, keep under control,
   keep under wraps, lag, lay by, lay under restraint, let alone,
   let down, let up, lose ground, lose momentum, lose speed,
   make late, make no sign, meddle, moderate, never let on,
   not give away, not tell, not touch, not use, obstruct, oppose,
   play dumb, preserve, prohibit, pull, pull in, put apart, put aside,
   put by, reef, refrain, refuse, rein, rein in, relax, repress,
   reserve, resist, restrain, retain, retard, retrench, save, save up,
   scotch, secrete, set apart, set aside, set back, set by, slack off,
   slack up, slacken, slow, slow down, slow up, snub, spare, stay,
   stop, straiten, suppress, take in sail, throttle down, veil, waive,
   withhold