まじ is a predecessor of まい, both archaic foorm of auxilliary verb meaning negation of べし.
In the modern Japanese, it is not commonly used in conversation excpet for some archaic set phrase such as すまじきものは宮仕え、This was written about 1,000 years ago by 清少納言(Seishonagon) in her essay called 枕草子(Makurano Soushi).