lightweight

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun One that weighs relatively little or less than average.
• noun A weight division in professional boxing having an upper limit of 135 pounds (60.7 kilograms), between junior lightweight and junior welterweight.
• noun A boxer competing in this weight division.
• noun A similar weight division in other sports, such as weightlifting.
• noun A contestant in this weight division.
• noun A person of little ability, intelligence, influence, or importance.
• adjective Weighing relatively little; not heavy.
• adjective Sports Of, relating to, or characteristic of a lightweight.
• adjective Having no significance or influence.

from The Century Dictionary.

• noun In sporting, a man or an animal of a certain weight prescribed by the rules, between that of the middle-weight on one hand and that of the feather-weight on the other; hence, any person of light weight or of comparatively little importance.

from the GNU version of the Collaborative International Dictionary of English.

• adjective Light in weight, as a coin; specif., applied to a man or animal who is a lightweight.
• adjective made of thin cloth, of relatively little weight; -- of clothing or fabrics.
• noun One of less than average weight
• noun Colloq., Chiefly U. S., Colloq., Chiefly U. S. A person of small importance or mental ability, especially one who is unimportant but cheeky and presumptuous.
• noun a wrestler who weighs 139-154 pounds.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun boxing A boxer in a weight division having a maximum limit of 135 pounds for professionals and 132 pounds for amateurs.
• noun bodybuilding A competitive weight division between bantamweight and welterweight, with specific weights varying by organization.
• noun One of little consequence or ability.
• noun A person who can't handle their drink.
• noun A political candidate with little chance of winning
• adjective Lacking in earnestness, ability, or profundity
• adjective Having less than average weight
• adjective Lacking in strength

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun someone who is unimportant but cheeky and presumptuous
• noun an amateur boxer who weighs no more than 132 pounds
• adjective weighing relatively little compared with another item or object of similar use
• noun a wrestler who weighs 139-154 pounds
• noun a professional boxer who weighs between 131 and 135 pounds
• adjective having no importance or influence