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?
1. the title given to someone high on the corporate ladder, in a position of authority, etc. who is also a douchebag.
2. a person who is not necessarily in authority over another but who is just a huge douchebag, i.e. on the scale of all douchebags instead of just on one ladder
Example 1: Jimmy is the arch douche of the accounting department.