To thresh is to harvest seeds from grain by beating or crushing it. Before the invention of machines to do this task, it took a huge amount of time to thresh grain by hand.
To make this tedious task go faster, farmers used to thresh in groups, throwing threshing bees where neighbors worked together. Special sticks called flails were used to beat the seeds out of the grain, and it took about an hour to thresh a bushel of wheat. The threshing machine was invented in the late 1700s, and today most farmers thresh using a combine harvester, which harvests and threshes the grain right in the field.