A thresher is a machine that separates grain from the rest of the plant after harvest. For thousands of years, farmers had to thresh wheat, rice, or barley by hand, beating the stalks with wooden tools called flails or having animals walk over them to knock the precious grains loose from the chaff (the dry husks and stems). This backbreaking work took days or weeks.

The invention of mechanical threshers in the 1700s and 1800s revolutionized farming. These machines could do in hours what took farmers weeks by hand. Early threshers were powered by horses walking in circles or by steam engines. Modern combine harvesters do the work of threshing right in the field while harvesting, finishing the entire job in one pass.

The word also refers to a type of shark called a thresher shark, named for its extraordinarily long tail fin that can be as long as its entire body. These sharks use their tails like whips to thresh through schools of fish, stunning their prey before eating them. It's a fitting name: just as a threshing machine separates grain with powerful mechanical action, the thresher shark uses its tail to separate a fish from its school.