The Collaborative International Dictionary of English v.0.48:

Stagger \Stag"ger\ (-g[~e]r), v. i. [imp. & p. p. Staggered
   (-g[~e]rd); p. pr. & vb. n. Staggering.] [OE. stakeren,
   Icel. stakra to push, to stagger, fr. staka to punt, push,
   stagger; cf. OD. staggeren to stagger. Cf. Stake, n.]
   1. To move to one side and the other, as if about to fall, in
      standing or walking; not to stand or walk with steadiness;
      to sway; to reel or totter.
      [1913 Webster]

            Deep was the wound; he staggered with the blow.
                                                  --Dryden.
      [1913 Webster]

   2. To cease to stand firm; to begin to give way; to fail.
      "The enemy staggers." --Addison.
      [1913 Webster]

   3. To begin to doubt and waver in purpose; to become less
      confident or determined; to hesitate.
      [1913 Webster]

            He [Abraham] staggered not at the promise of God
            through unbelief.                     --Rom. iv. 20.
      [1913 Webster]

WordNet (r) 3.0 (2006):

staggering
    adj 1: so surprisingly impressive as to stun or overwhelm; "such
           an enormous response was astonishing"; "an astounding
           achievement"; "the amount of money required was
           staggering"; "suffered a staggering defeat"; "the figure
           inside the boucle dress was stupefying" [syn:
           astonishing, astounding, staggering, stupefying]

Moby Thesaurus II by Grady Ward, 1.0:

151 Moby Thesaurus words for "staggering":
   amazing, ambling, ambulation, astonishing, astounding, backpacking,
   breathtaking, broken, capricious, careening, catchy, cautious,
   choppy, circumspect, claudicant, confounding, crawling, creeping,
   creeping like snail, deliberate, desultory, deviative,
   disconnected, discontinuous, easy, eccentric, electrifying,
   erratic, eye-opening, faltering, fitful, flagging, flickering,
   fluctuating, foot-dragging, footing, footing it, footwork, gentle,
   going on foot, gradual, guttering, halting, herky-jerky,
   heteroclite, hiking, hitchhiking, hitching, hobbled, hobbling,
   hoofing, idle, immethodical, inconstant, indolent, intermittent,
   intermitting, irregular, jarring, jerky, jolting, languid,
   languorous, lazy, legwork, leisurely, limping, lumbering, lurching,
   marching, mind-boggling, miraculous, moderate, nonuniform,
   overwhelming, patchy, pedestrianism, perambulation, poking, poky,
   rambling, relaxed, reluctant, rough, sauntering, scrappy, shocking,
   shuffling, slack, slothful, slow, slow as death, slow as molasses,
   slow as slow, slow-crawling, slow-foot, slow-going, slow-legged,
   slow-moving, slow-paced, slow-poky, slow-running, slow-sailing,
   slow-stepped, sluggish, snail-paced, snaillike, snatchy, spasmatic,
   spasmic, spasmodic, spastic, spectacular, sporadic, spotty,
   startling, strange, strolling, stunning, surprising, tentative,
   thumbing, thumbing a ride, toddling, tortoiselike, tottering,
   tramping, treading, trudging, turtlelike, uncertain, unequal,
   uneven, unhurried, unmethodical, unmetrical, unregular,
   unrhythmical, unsettled, unsteady, unsystematic, variable, veering,
   waddling, walking, wandering, wavering, wobbling, wobbly,
   wonderful, wondrous