I received an e-mail from an acquaintance and she used the word bestie; I had no idea what she was talking about. It means, best friends. A fairly new expression:
A person’s best friend; a very close friend. Cf. best friend n. 1.
1991 Observer 16 June 20/6 Diana’s friends often date from the days BC (Before Charles). Some are Besties—reliable pals from school.
According to Urban dictionary, it’s originally British slang. My vocabulary is quite extensive, but I’m having trouble keeping up with every neologistic expression. Perhaps my problem is that I rarely read modern literature and neologisms in the classics--which are far fewer than today’s newly coined words-- are now well established in the English lexicon.