Lightweight \Light"weight`\, n. One of less than average weight; specif.:
   (a) In boxing, wrestling, etc., one weighingnot more than 133 pounds (U. S. amateur rules 135 pounds, Eng. 140 pounds).
   (b) A person of small impotance or mental ability. [Colloq., Chiefly U. S.]

Source: Webster's Revised Unabridged Dictionary (1913)
 

Lightweight \Light"weight`\, a. Light in weight, as a coin; specif., applied to a man or animal who is a lightweight.

Source: Webster's Revised Unabridged Dictionary (1913)