Someone who is never able to think on their feet, but can always think of the perfect course of action to have taken well after the event has transpired. Often eager to tell people what they "should have done".
John: I should not have told her I screwed her sister until after she signed the divorce settlement.
Joe: Way to be hindsight savvy.
Jim: You should have folded on the flop.
Jack: Go fuck your self Jim, you pompous ass, hindsight savvy, mother fucker.