The Collaborative International Dictionary of English v.0.48:

Lightweight \Light"weight`\ (l[imac]t"w[=a]t`), n.
   One of less than average weight; specif.: In boxing,
   wrestling, etc., one weighing not more than 133 pounds (U. S.
   amateur rules 135 pounds, Eng. 140 pounds). [WordNet sense 1]
   [Webster 1913 Suppl.]

   2. A person of small importance or mental ability, especially
      one who is unimportant but cheeky and presumptuous.
      [WordNet sense 2] [Colloq., Chiefly U. S.]

   Syn: whippersnapper, jackanapes.
        [Webster 1913 Suppl. + WordNet 1.5]

   3. a wrestler who weighs 139-154 pounds. [WordNet sense 3]
      [WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

Lightweight \Light"weight`\, a.
   1. Light in weight, as a coin; specif., applied to a man or
      animal who is a lightweight.
      [Webster 1913 Suppl.]

   2. made of thin cloth, of relatively little weight; -- of
      clothing or fabrics; as, lightweight fabrics. [WordNet
      sense 1]
      [WordNet 1.5]

WordNet (r) 3.0 (2006):

lightweight
    adj 1: weighing relatively little compared with another item or
           object of similar use; "a lightweight fabric";
           "lightweight wood"
    2: having no importance or influence; "a lightweight intellect"
    n 1: a professional boxer who weighs between 131 and 135 pounds
    2: someone who is unimportant but cheeky and presumptuous [syn:
       whippersnapper, jackanapes, lightweight]
    3: an amateur boxer who weighs no more than 132 pounds
    4: a wrestler who weighs 139-154 pounds

Moby Thesaurus II by Grady Ward, 1.0:

187 Moby Thesaurus words for "lightweight":
   Boeotian, Chinese boxer, Milquetoast, a nobody, a nothing, baby,
   bantam, bantamweight, banty, big baby, black belt, block, boxer,
   breakable, brittle, brown belt, bruiser, button, catchweight,
   cheap-jack, chicken, chit, cipher, clod, cobwebby, cog, common man,
   commonality, commonalty, creature, crumbly, crybaby, dainty,
   delicate, delicately weak, diminutive, dimwit, dolt, donkey,
   doormat, dope, dull tool, dullard, dumb cluck, dumbbell, dummy,
   dunce, effeminate, featherweight, fighter, figurehead, fingerling,
   fisticuffer, flimsy, flunky, flyweight, follower, forceless,
   fragile, frail, frangible, gimcrack, gimcracky, gossamery, gowk,
   gutless wonder, heavyweight, hoi polloi, imponderous, impotent,
   ineffective, ineffectual, inefficacious, inferior, insignificancy,
   invertebrate, jackstraw, jellyfish, jerry, jerry-built, jobbernowl,
   junior, karate expert, lackwit, lamebrain, light,
   light heavyweight, little fellow, little guy, looby, loon,
   lower class, lower orders, man of straw, masses, mediocrity,
   meek soul, middleweight, milksop, mini, minikin, minnow, minny,
   mollycoddle, mouse, namby-pamby, nebbish, niais, nincompoop, ninny,
   ninnyhammer, nitwit, no-account, nobody one knows, noddy,
   nonentity, nubbin, obscurity, of no account, palooka, pansy,
   pantywaist, papery, pasteboardy, pawn, peewee, pip-squeak, pony,
   powerless, prizefighter, pug, pugilist, punk, puny, pushover, put,
   runt, sad sack, savate expert, scrub, second fiddle, secondary,
   shattery, shrimp, sissified, sissy, sleazy, slight, slip,
   small fry, small potato, small potatoes, snip, snippet, softling,
   softy, sop, sparrer, squirt, squit, stupid, subaltern, subordinate,
   tacky, thickwit, third stringer, tit, unauthoritative, underling,
   understrapper, underweight, unheavy, unsubstantial, wart, weak,
   weak sister, weakling, weightless, welterweight, whiffet,
   whippersnapper, wisp, wispy, without any weight, witling, womanish,
   yes-man



The Jargon File (version 4.4.7, 29 Dec 2003):

lightweight
 adj.

    Opposite of heavyweight; usually found in combining forms such as
    lightweight process.