When I first started working on the KCBT back in 1998, the term "ginzy" I heard jokingly tossed around a lot. Thinking about it a bit more, some loudmouth would jokingly yell "do a ginzy" when two brokers quoted a tight market with size on both sides. For years I never knew what it was or the origin of the term but later reread The New Gatsby's and it noted what it actually was and rather that quote from the book, here is the definition from investopedia:
"Ginzy trading is an order of different prices placed by a floor broker. It occurs when a floor broker attempts to avoid an exchange's rule against trading at fractional increments, often called "split ticks". Ginzy trading works when a floor broker executes a particularly large order and fills a portion of the order at one price, but then fills the remainder of the order at a different price. Hence, the floor broker has quoted different prices to different customers on the same order. This not only is considered unethical; it also is illegal. Some exchanges have technology systems in place to prevent these illegal trades, but many smaller exchanges do not have such strategies in place. Ginzy trading is in violation of the Commodity Exchange Act as set forth by the United States Commodity Futures Trading Commission."
A real world example would be a market, which trades in one point increments, of 5 bid at 6 offer so the participants would collude to do half 5s and half 6s so the average price would be 5.5 which both participants would be satisfied with, basically splitting the tick.
I never saw anyone do it but apparently it would happen and the origin of the term "ginzy" came from a trader named Ginsburg who chronically did it.
*UPDATE* The following comment was posted anonymously in the comment section and warrants a bump up to the main post:
"There are instances where something commonly referred to as a "Ginzy" is perfectly legal, an example would be a market that's 5 bid at 7, a broker gets an order to sell 50 at the market, there are 5 locals bidding 5, if the broker offers 50 at 6 and one local steps up and buys 25 lots at 6, knowing that if nobody joins him buying 6s he would get the remainder of the 50 lot at 5, that would be legal (the tactic would be based on the knowledge of the broker's normal practice of handling orders in that fashion, and avoids specific collusion) and really a normal part of pit business (for the broker the incentive of only selling 5s to the local[s] who also paid 6 is to get better fills for the customer through a split fill and for the local it's to get 25 at the bid). I've heard that scenario also described as the local doing a "Ginzy".
The above scenario may also result in the entire order being filled at 6 when one of the other 4 locals decides that he doesn't want that so and so who stands next to him all day to buy 5s if he's not buying 5s."