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4.
1- A person you consider your friend, but not your most favorite person.

2- secondary bestfriend.
"I was at the mall the other day and saw my bff jill. i was dreding every second of that hideous conversation."
by Kay Luh September 11, 2007
 
1.
In reference to Best friend forever, as in the commercial...
Mom: Who are you texting so much?
Girl: I DK, my BFF Jill

Anyone you hang around a lot, for work related reasons or otherwise, even if they aren't your best friend.
RIck: AnnMarie, where is your BFF Jill?
AnnMarie: I don't know. I feel lost without her, we've got to get this project done for Monday.
by Lanny123 June 16, 2007
 
2.
hold out your right hand in front of your face, fingers spread out, palm away - the fingers of your hand should spell out "jill" respectively; referred to when someone is suspected of masturbating
you were in your room for awhile. spending quality time with your bff jill?
by dulcedelecheistheshiit December 16, 2008
 
3.
your best friend forever
sometimes named juill
from the cingular texting commercial

mom: " you have you been texting 50 times a day?!"
stephanie: "idk, my bff jill?"
by clairerocks June 12, 2007