In short, it means No. It is derived from the word "Negatory", which is derived from the word "Negative". Simply adding the "ma" before the "tory" just adds flavour to the word. Also, you know you are godly if you use this term.
It is mainly used in online gaming.
Used in real life for humour purposes.
"Rush mid with me"
"Matt take me to McDonalds"