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A person who was once someone's closest friend, but who became their worst enemy.
The pain received from having the worst enemy is worse, because of the painful loss of a friend.
Both parties usually become bitter and obsessive.
Josh: What? Why aren't you with Sara? You're always with her!
Jade: Oh, she's my Arch-Ex-Friend now. Haven't you seen her bitter attacks on Facebook?
by Reformed August 13, 2010