To describe something ultra best. Instead of saying something like, "that was awesome", or "wow that's the coolest thing I have ever seen". Also, can be used to describe besting something, or dominating it. Opposite of the word being 'worstachire'.
1 Mike - "dude, that chick I met the other night at the bar
asked me out!"
Seth - "bestachire!"
2 Matt - "I just bestachired Worms Armageddon online!"