A thresher is the thing or person that threshes — that beats cut grain to knock the seeds loose from the stalks. Most often it means the MACHINE. In the 1800s the steam-powered threshing machine changed farm life completely, doing in a day what a village once did all winter, and "thresher" usually points to that kind of machine or to the threshing part inside a modern combine harvester. It can also mean the worker doing the job by hand, though that use is old. There is a second, quite different meaning worth knowing: a thresher (or thresher shark) is a shark with an enormously long upper tail, as long again as its body, which it swings like a whip to stun the fish it hunts. It got the name because that swinging tail looked to sailors like a flail beating grain. So one word, two pictures: a machine in a field, and a shark in the sea — joined by the idea of something long swinging down hard.