The Latin word for rustler. An abigeus, thus, commits abigeat - livestock theft. Not just any rustler earned the name abigeus, though; you had to more or less make a living at it, stealing large amounts of livestock (primarily cattle, natch) constantly. Simply stealing your neighbor's horse wasn't worthy of the title; you had to steal all of your neighbor's horses to count.
The word comes from the Latin abigere, meaning to "drive", and in literal terms means "one who drives."
It has been for the most part replaced by the (slightly) more modern word abactor for legal purposes.