Definition
A player, coach or manager who verbally annoys and distracts opposition players and umpires from his team's dugout bench.
Origins
As verbal jousting is frequently called "riding", the "rider" sitting on a bench came to be known as the "bench jockey".
How People Actually Use It
A negative way of describing someone in the scientific field who spends a vast majority of his or her time in front of a bench or lab station performing mundane and repetitive tasks. The typical entry level position for college graduates who enter a science job and how disgruntled lab techs refer to themselves. Those who do well are typically promoted to a supervisory or managerial position before going on to get an advanced degree such as an MBA or PhD.
"Billy got a bachelors in biology and his first job was working as a low paid bench jockey. Brian got a masters in biology but still had to start off as a higher paid bench jockey. Sue got a bachelors in biology and like her colleagues had to start as a bench jockey. After a few years she left to get her MBA and now oversees all of her company's bench jockeys."
Source: Urban Dictionary