No translation is given in Benson, nor Wells, nor Vikivortaro. Here is the link to the entry in the free online dictionary of Merriam-Webster: https://www.merriam-webster.com/dictionary/reiver
I couldn't find anything either for reive (nor reave), but the word
comes to mind, which with rab/ expresses the looting of cattle or goods and with atak/ the suddenness and violence of it. There are also sereval entries in Tekstaro of forms of this word.
So reiver would be rabatakanto or rabatakisto.