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?
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