shekel: the basic Israelite unit of weight, about eleven grams, used both to weigh things and to measure out silver as money.
A shekel is first of all a weight, not a coin. It is the standard small unit on the Israelite scale, roughly eleven grams, and almost anything can be measured by it: silver and gold, bronze and iron, even a man's hair or a daily ration of food. The word comes straight from the verb meaning "to weigh," and that is what people did with it. Larger amounts were counted in minas and talents, and the shekel was the everyday unit underneath them.
Because Israel before the exile had no minted coins, the same weight did double duty as money. To buy a field, free a slave, pay a fine, or give to the temple, people weighed out so many shekels of silver on a balance. So the two sides of the word, a measure of weight and a sum of money, are not two different words but one unit seen from two angles: silver was money precisely because it was weighed. Abraham weighing out four hundred shekels to buy a burial plot, and Jeremiah weighing out seventeen shekels on the scales just before Jerusalem fell, are doing the very thing the word describes.
The law often ties the figure to a fixed reference, the shekel "of the sanctuary," so that the standard stays honest, and the prophets attack merchants who shrink the shekel to cheat the poor. Reading it as a modern coin or converting it straight into dollars misses the point: a shekel was a weight on a scale, and its whole role in the law was a measure everyone could trust.