Someone who believes they've got the answer to every question even if the question hasn't been asked or if they really don't have the answer (... but of course they believe they do). This is not a shy individual, but rather someone whose ego is over flowing primarily through their mouth, but seems to have come from the other end of their digestive system.
Standing in the door of his single wide, Mr. know it all gave me his latest real estate tips.
Mr. know it all told the CIA there were weapons of mass destruction in Iraq. Unfortunately, they were listening.