The root of neshech [נֶשֶׁךְ] is nashach, [נָשַׁךְ ], which relates to the act of biting. And the root of tarbit [תַרְבִּית] is rav [רב] or harbeh [הרבה], which relates to the idea of "many" or "increase."
In the Kli Yakar's words:
And it mentions the term "neshech and tarbit" because in relation to the borrower, it's called neshech because the interest [ribit] is similar to a snake that comes upon the way and bites [noshech] one's heel, producing a small scratch which then goes and spreads and becomes swollen up to one's head. But initially, it's not evident until its venom increases beyond cure.
And it says, "and your brother shall live with you" because you, too, will surely live as it states in Yechezkel 18:13:
"He gives with neshech and takes with tarbit - and he shall live? He shall not live out all these abominations...."
For he who deprives another of livelihood - it is as if he killed him.
Therefore, he becomes one who gives life, yet doesn't live.
However, regarding he who assists with another's subsistence, the din [verdict] is that he gives life and also lives.
Therefore it says, "and your brother shall live with you."
But when we help one another, we are also helping ourselves in the most profound way possible.
This is my own translation and any errors are also mine.