1. Short for retard.
2. Also used in Greek language most likely as an insult or get attention.
3. When used in letters or emails: Abbreviation of the word regarding.
4. If extended e's are involved: this is the battle cries of an outsider in trouble because of normies.
Ex 1: "Jack eat all my potato salad when I told him not too. What a fucking re."
Ex 2: "Re, don't fall down that hole in front of you."
Ex 3: RE: Car for sale.
Ex 4: "DEAD MEME YOU FUCKING NORMIE! REEEEEEEEEEEEEEEEEEEEEEEEEE!!!"