The definition of muckleboob is big idiot. The word itself is defined by combining Muckle which is an ancient scottish word meaning "Large amount" and boob, which is a dullard, oaf, or useless person. It can also be used to describe a disasterous outcome.
Personal Description: That guy is a real Muckleboob
Outcome: This job should have been easy, but now it's a real Muckleboob.