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

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

lightweight
 adj.

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