In retrospect, it's easy to see why folks who use hindsight to criticize what others have done are known as Monday-morning quarterbacks or simply Monday quarterbacks: since most American football games are played on weekends, the would-be omniscient critic needs to wait til Monday morning before he or she can call plays accurately.
Eventually, the idea of critically rehashing an already played game inspired some now-forgotten fan to kick that term off the gridiron and into the stands. Both spectators and folks who avoid contact sports have run with the idea of using Monday morning quarterback as a synonym for second-guesser.

Can you guess the year Monday morning quarterback first appeared in print? It was 1941, the same year game plan made its non-sports debut. That term originated in sports (such as football) that rely on strategic plans, but game plan soon found a place among non-athletes receptive to a term meaning a strategy for achieving an objective.