Either
a) A shortened version of 'best
friends' for internet talk.
or
b) Someone who is really close to being categorized as a best
friend, but not exactly one yet.
a) TTYL bestie! XOXOX lylas!
b) Jamie: You know you're my bestie for life, girl.
Arianna: WHAT, I'M NOT YOUR BEST
FRIEND!!?! WHAT KIND OF
RELATIONSHIP IS THIS?