The factorials of negative integers have no defined meaning.

We know that factorials satisfy

 x*(x-1)! = x!

However, if there is a (-1)!, then we would able to write:

      x*(x-1)! = x!

=> 0*(0 - 1)! = 0!       {Put x = 0}

=> 0*(-1)! = 0!

=> 0 = 1                 {since 0! = 1}

Which is not possible.

So, the factorials of negative integers have no defined meaning.