A bean, probably of the carob tree, the smallest weight, and also the smallest piece of money, among the Hebrews, equal to the twentieth part of a shekel (Exodus 30:13; Leviticus 27:25; Numbers 3:47). This word came into use in the same way as our word "grain," from a grain of wheat.

Noah Webster's New International Dictionary of the English Language

(n.) A small coin and weight; 1-20th of a shekel.