There are many hundred or thousands of shortened words in the English language,most people who have existed for a few years get to know them.Those who don't understand one ask politely and go on there way.
As they say on South Park "I learned something today."
MBD or mbd is one of the most common ones on PC forums.
Yes its a MotheBoarD also known as Mainboard,or system board.
Mother is often called Mom.Have you figured this out yet and Daddy is Dad. ?
Who has the answer to that ?