Definitions of lightweight:

• noun:   a professional boxer who weighs between 131 and 135 pounds
• noun:   a wrestler who weighs 139-154 pounds
• noun:   an amateur boxer who weighs no more than 132 pounds
• noun:   someone who is unimportant but cheeky and presumptuous
• adjective:   having no importance or influence

  Example: "A lightweight intellect"

• adjective:   having relatively little weight

  Example: "Lightweight fabrics"