It's related to the phrase "Monday morning quarterback"... in American football, the professional teams usually play on Sunday afternoons during season. The discussion around the watercooler on Monday morning consists of various opinions on what the quarterback should have done to win the game. Second guessing is the same genre'... a person that has a different opinion on something, after the fact making his opinion fool proof since hindsight is always 20/20...
any idea what the words actually mean, Clanad? Does it mean the first guess is the one actually made at the time and the second the one made much later? ('Monday morning quarterback' is clear enough.)
You're on the right track... it has the connotation of the person uttering it in effect, saying, "I made my decision/choice on the the information I had at the time. Now, here you are making a better choice because, now, you have more or all the information neccessary to make a better decision/choice...
I usually hear this complaint, "I don't want to be second guessed", made by political leaders. The reason this is often a bogus complaint, is that factual information was available at the time of their "first guess", they just chose to ignore it.
Sorry, I don't know the origin but my understanding of the term differs from the others.
I have always heard it used in the context that someone is anticipating a persons reactions or reasons, usually innapropriately. As such, it takes place before a decision is made, not after.