The Collaborative International Dictionary of English v.0.48:

Impulsive \Im*pul"sive\, a. [Cf. F. impulsif.]
   [1913 Webster]
   1. Having the power of driving or impelling; giving an
      impulse; moving; impellent.
      [1913 Webster]

            Poor men! poor papers! We and they
            Do some impulsive force obey.         --Prior.
      [1913 Webster]

   2. Actuated by impulse or by transient feelings.
      [1913 Webster]

            My heart, impulsive and wayward.      --Longfellow.
      [1913 Webster]

   3. (Mech.) Acting momentarily, or by impulse; not continuous;
      -- said of forces.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Impulsive \Im*pul"sive\, n.
   That which impels or gives an impulse; an impelling agent.
   --Sir W. Wotton.
   [1913 Webster]

WordNet (r) 3.0 (2006):

impulsive
    adj 1: proceeding from natural feeling or impulse without
           external stimulus; "an impulsive gesture of affection"
           [syn: impulsive, unprompted]
    2: without forethought; "letting him borrow her car was an
       impulsive act that she immediately regretted"
    3: having the power of driving or impelling; "a driving personal
       ambition"; "the driving force was his innate enthusiasm"; "an
       impulsive force" [syn: driving, impulsive]
    4: determined by chance or impulse or whim rather than by
       necessity or reason; "a capricious refusal"; "authoritarian
       rulers are frequently capricious"; "the victim of whimsical
       persecutions" [syn: capricious, impulsive, whimsical]
    5: characterized by undue haste and lack of thought or
       deliberation; "a hotheaded decision"; "liable to such
       impulsive acts as hugging strangers"; "an impetuous display
       of spending and gambling"; "madcap escapades"; (`brainish' is
       archaic) [syn: hotheaded, impulsive, impetuous,
       madcap, tearaway(a), brainish]

Moby Thesaurus II by Grady Ward, 1.0:

238 Moby Thesaurus words for "impulsive":
   a bit previous, abrupt, actuating, adrift, advanced, afloat,
   alternating, amorphous, animating, automatic, blind, breakneck,
   breathless, brittle, capricious, causal, causative, changeable,
   changeful, changing, choppy, compelling, compulsive, conditioned,
   corruptible, deciduous, desultory, deviable, deviating, deviative,
   deviatory, devil-may-care, different, directive, disorderly,
   divaricate, divergent, diversified, diversiform, dizzy, driving,
   dying, eccentric, electrifying, emotional, ephemeral, erose,
   erratic, evanescent, extemporaneous, fading, far ahead,
   fast and loose, fickle, fitful, fleeting, flickering, flighty,
   flitting, fluctuating, fly-by-night, flying, foolhardy, forced,
   forward, fragile, frail, freakish, fugacious, fugitive, giddy,
   half-baked, half-cocked, hasty, headlong, ill-considered,
   immediate, impatient, impellent, impelling, impermanent, impetuous,
   inconsistent, inconstant, indecisive, inducive, infirm, inherent,
   innate, instinctive, insubstantial, involuntary, irregular,
   irresolute, irresponsible, jagged, jerky, libidinal, madcap, mazy,
   mechanical, mercurial, momentary, moody, mortal, motivating,
   motivational, motive, motley, moving, mutable, natural,
   nerve-shattering, nonconformist, nondurable, nonpermanent,
   nonstandard, nonuniform, not firm, offhand, overhasty, oversoon,
   panting, passing, perishable, pluralistic, precipitant,
   precipitate, precipitous, precocious, premature, pressing,
   previous, pulsive, quick, ragged, rambling, rash, reckless, reflex,
   reflexive, restless, rough, roving, rushed, scatterbrained,
   shapeless, shifting, shifty, shocking, short-lived, shuffling,
   snap, spasmodic, speedy, spineless, spontaneous, sporadic,
   spur-of-the-moment, startling, subliminal, sudden, surprising,
   swift, temporal, temporary, thrusting, too early, too soon,
   transient, transitive, transitory, unaccountable, unanticipated,
   uncertain, unconscious, unconsidered, uncontrolled, uncrystallized,
   undependable, undisciplined, undurable, unenduring, unequable,
   unequal, uneven, unexpected, unfixed, unforeseen, unintentional,
   unjelled, unlearned, unlooked-for, unmatured, unmeditated,
   unorthodox, unplanned, unpredictable, unpredicted, unpremeditated,
   unprepared, unprompted, unreliable, unrestrained, unripe,
   unsettled, unstable, unstable as water, unstaid, unsteadfast,
   unsteady, unsystematic, unthinking, untimely, ununiform, unwilled,
   unwilling, unwitting, urgent, vacillating, vagrant, variable,
   variegated, variform, various, varying, vicissitudinary,
   vicissitudinous, volatile, wandering, wanton, wavering, wavery,
   wavy, wayward, whimsical, wild, wishy-washy