The word probably comes from μωρέ, the vocative of the ancient Greek μωρός 'dull, sluggish, foolish, stupid'.
re is often accompanied by a slang word but generally speaking you can use it any given sentence.
I told you a 1000 times re I do not do this stuff anymore.
Get me a coffee re.
Re is a latin word (pronounced 'ray') that means 'in the matter of' and was adopted by the English courts along with many other latin phrases. It has since moved into general correspondence.