Sweat is perspiration that makes you feel cooler when it evaporates off the surface of your skin — or the giving-off of this liquid. If you exercise hard, you will sweat.
The word sweat comes from the Old English word swætan, which means “perspire,” and “work hard.” Though sweat is the body's way of cooling itself, today it also means "to worry or be stressed out." If someone tells you not to sweat it, it means "don't worry." But if someone is "sweating" you, it means you are hassled, maybe being asked every five minutes, "Did you finish your work yet?"