also: のみや
noun
Refers to an illegal or underground bookmaker, especially for horse racing or other gambling. The term comes from the image of a small-time operator (ノミ = flea).
彼は競馬のノミ屋を営んでいた。
He ran an illegal bookmaking operation for horse racing.
noun
bucket shop; trader performing bucketing
In stock market context, a broker who takes the opposite side of a customer's trade instead of executing it on the market, or the shop itself. Related to ノミ行為.
See also: ノミ行為 (ノミこうい)
その証券会社はノミ屋として摘発された。
That securities firm was exposed as a bucket shop.
ノミ行為 is the act of bucketing, while ノミ屋 refers to the person or shop that performs it.
From ノミ (flea), implying a small, parasitic operation, plus 屋 (shop/person). The exact origin is uncertain, but it's associated with illegal, small-scale gambling or stock trading.