Jitney

A dance popular in the 1920s, until Charles Lindbergh tried to do it on the top of a flagpole and fell off. The great jitney scandal of 1928.

In general terms, the word "jitney" refers to a either a small bus or an unlicensed taxi. The word has been applied to the stock market for a broker who bends the rules in particular ways.

You're a stock broker with access to the NYSE. Your brother runs a fly-by-night boiler-room trading business. He doesn't have NYSE access, but runs his trades through you. An example of a jitney.

Another connotation: your brother suggests that the two of you trade stocks back and forth. The increased volume will generate additional commissions, even though each of you only holds the stock for a few seconds. Doing that would be another example of a jitney.