If thou lend money to any of my people that is poor by thee, thou shalt not be to him as an usurer, neither shalt thou lay upon him usury. Neither shalt thou lay upon him usury - נשך neshech, from nashach, to bite, cut, or pierce with the teeth; biting usury. So the Latins call it usura vorax, devouring usury. "The increase of usury is called נשך neshech, because it resembles the biting of a serpent; for as this is so small as scarcely to be perceptible at first, but the venom soon spreads and diffuses itself till it reaches the vitals, so the increase of usury, which at first is not perceived nor felt, at length grows so much as by degrees to devour another's substance." - Leigh.
It is evident that what is here said must be understood of accumulated usury, or what we call compound interest only; and accordingly נשך neshech is mentioned with and distinguished from תרביה tarbith and מרביה marbith, interest or simple interest, Leviticus 25:36, Leviticus 25:37; Proverbs 28:8; Ezekiel 18:8, Ezekiel 18:13, Ezekiel 18:17, and Exodus 22:12 - Parkhurst.
Perhaps usury may be more properly defined unlawful interest, receiving more for the loan of money than it is really worth, and more than the law allows. It is a wise regulation in the laws of England, that if a man be convicted of usury - taking unlawful interest, the bond or security is rendered void, and he forfeits treble the sum borrowed. Against such an oppressive practice the wisdom of God saw it essentially necessary to make a law to prevent a people, who were naturally what our Lord calls the Pharisees, φιλαργυροι, lovers of money, (Luke 16:14), from oppressing each other; and who, notwithstanding the law in the text, practice usury in all places of their dispersion to the present day.