The word originates in from the Irish 'maith' which means good. Caillín maith meaning 'good girl' became a way of refering to your girl friend.
Over the years the word changed so it was simply 'mot', which became used by lower class Dubliners.
Now is widely used to refer to all women of similar ages and not just girlfriends.
The thing that makes a mot different from a normal shit is the size. This shit is very big. Bigger than your usual shit. It is very smelly too. You know you are taking a mot when it is very difficult to get out.
"Jeg har lige været ude at trykke en kæmpe mot."
"I have justtaken a giant mot"