The word "reiver" is an old Scottish term that refers to a person who steals cattle or other livestock. If you are looking for synonyms for this word, you may want to consider using the words rustler, cattle thief, or cattle wrestler. Another synonym for the word reiver could be the word raider, which is typically used to describe someone who attacks and takes over a territory or land. Other possible synonyms that can be used include the words bandit, robber, marauder, and plunderer. All of these words capture the essence of the word reiver and refer to people who engage in unethical and illegal activities.