The Collaborative International Dictionary of English v.0.48:

Rampant \Ramp"ant\ (r[a^]mp"ant), a. [F., p. pr. of ramper to
   creep. See Ramp, v.]
   1. Ramping; leaping; springing; rearing upon the hind legs;
      hence, raging; furious.
      [1913 Webster]

            The fierce lion in his kind
            Which goeth rampant after his prey.   --Gower.
      [1913 Webster]

            [The] lion . . . rampant shakes his brinded mane.
                                                  --Milton.
      [1913 Webster]

   2. Ascending; climbing; rank in growth; exuberant.
      [1913 Webster]

            The rampant stalk is of unusual altitude. --I.
                                                  Taylor.
      [1913 Webster]

   3. (Her.) Rising with fore paws in the air as if attacking;
      -- said of a beast of prey, especially a lion. The right
      fore leg and right hind leg should be raised higher than
      the left.
      [1913 Webster]

   Rampant arch.
      (a) An arch which has one abutment higher than the other.
      (b) Same as Rampant vault, below.

   Rampant gardant (Her.), rampant, but with the face turned
      to the front.

   Rampant regardant, rampant, but looking backward.

   Rampant vault (Arch.), a continuous wagon vault, or cradle
      vault, whose two abutments are located on an inclined
      plane, such as the vault supporting a stairway, or forming
      the ceiling of a stairway.
      [1913 Webster]

WordNet (r) 3.0 (2006):

rampant
    adj 1: unrestrained and violent; "rampant aggression"
    2: rearing on left hind leg with forelegs elevated and head
       usually in profile; "a lion rampant" [syn: rampant(ip),
       rearing]
    3: (of a plant) having a lush and unchecked growth; "a rampant
       growth of weeds"

Moby Thesaurus II by Grady Ward, 1.0:

194 Moby Thesaurus words for "rampant":
   abandoned, abounding, abundant, affluent, all-sufficing, ample,
   anabatic, aplenty, ascendant, ascending, ascensional, ascensive,
   average, besetting, bolt upright, bottomless, bounteous, bountiful,
   climbing, common, copious, current, diffuse, disobedient,
   disorderly, dominant, downright, effuse, elevated, epidemic, erect,
   everywhere, exalted, excessive, exhaustless, extravagant,
   exuberant, fat, fertile, flourishing, flush, frenzied, full,
   galore, generous, go-go, headstrong, heady, high, immoderate,
   in control, in plenty, in quantity, in the ascendant, incontinent,
   indiscriminate, indulgent, inexhaustible, inordinate,
   insubordinate, intemperate, irrepressible, irresponsible, lavish,
   lawless, lax, leaping, liberal, licentious, lifted, lofty, loose,
   luxuriant, many, maximal, mounting, much, mutinous, nonrestrictive,
   normal, numerous, obstreperous, on stilts, opulent, ordinary,
   out of control, out of hand, overflowing, pandemic, permissive,
   plenitudinous, plenteous, plentiful, plenty, popular, predominant,
   predominating, prevailing, prevalent, prodigal, productive,
   profuse, profusive, raised, rank, rearing, regnant, reigning,
   reinless, replete, revolutionary, rich, rife, riotous, rising,
   routine, ruling, running, running over, running wild, saltatory,
   scandent, scansorial, self-willed, skyrocketing, spiraling,
   springing, stand-up, standard, standing up, stereotyped, stilted,
   sublime, superabundant, teeming, terrorist, unaccountable,
   unbounded, unbridled, unchecked, uncoerced, uncompelled,
   unconstrained, uncontrollable, uncontrolled, uncurbed,
   undisciplined, unforced, ungoverned, uninhibited, unmastered,
   unmeasured, unmuzzled, unreined, unrepressed, unreserved,
   unrestrained, unrestrictive, unruly, unsubdued, unsuppressed,
   uparching, upcast, upcoming, upended, upflung, upgoing, upgrade,
   uphill, uphillward, uplifted, upraised, upreared, upright,
   uprising, upsloping, upstanding, upthrown, upward, upwith, usual,
   vertical, violent, wanton, wealthy, well-found, well-furnished,
   well-provided, well-stocked, wholesale, widespread, wild, wildcat,
   willful