Abarim Publications' online Biblical Hebrew Dictionary
The verb רשש (rashash) means to break down or shatter. It occurs in Syriac with the meaning of to grind.
In the Bible this verb occurs only two times: in Jeremiah 5:17 ("beat down thy fortified cities"), and Malachi 1:4 ("we are beaten down").
There are no derivatives of this verb extant in the Bible.