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A person who is a friend, only when your other friends are not present.

Also referred to as a "number two". Any social networking site has a Top Friends, and the number is your next best friend.
"Well, John isn't here, so I guess I'll have to talk to my fill-in friend."

"Amy had to go to the concert, so I guess it's just me and the number two."
by NOT a fill-in friend. May 03, 2009
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