A word that means best friends. Is usually said by teenage girls who are insecure about themselves and therefore calls everyone they come in contact with a "besty." She will generally comment "besties," on every picture of herself with someone else tagged on facebook.
The word is pronounced "best-ees". It rhymes with testies.
Insecure girl: Hi!!!
Insecure girl's friend: Who was that?
Insecure girl: Oh that was Jill, we're besties.