One of those words that can be used for any occasion when you don't know the right word to say. Most often used as an adjective, but it could fit for anything else, really.
Better than "best" friends.
A bond between two people who have overcome more than their fair share of obstacles and still maintained their friendship. Besterfriends tend to be quirky, and make even doing nothing a good time. They take too many pictures and laugh often too hard.
"Wow, they just got hit on by two guys in an ice cream truck. They must be besterfriends"
A person that never judges me, that never pretends to be someone she's not around me, I don't try to be anyone else around her either. A person that is never embarrassed by my weirdness and just acts weird along with me.
"Bestie Intuition" is some sort of higher 'knowledge based' power between two or more best friends where in everyday life or conversation, one can guess or even predict the thought or action-plan of another bestie before they say or do the thing that they had planned in there mind..
This "power" or "sixth sense" is often found between twins or extremely close siblings.
Example:
Ben - "hey man, whats doin'?"
Mitch - "oh hey man, nothin' much, just thort i'd have some food, i was thinking n....."