Okay - googled and this is what I found on a thoroughbred website.
By definition, "pinhooking" is a term used to describe the activities of an individual who hangs around the entrance of a livestock yard with the intention of buying livestock from incoming farmers. The purpose was to buy animals from the farmer for less money than they could get from the auction inside. This was a very common practice and still exists at smaller livestock yards in rural areas. The IRS has curtailed this practice at large livestock auctions; however, the practice still exists to some extent. Sometimes, the "pinhookers" would make money, sometimes they would lose. Generally, the transaction was not recorded, and therefore, the IRS did not get their share of any profits.
In horse circles it means buying a weanling or yearling to resale at a later date.
