"Armchair quarterback" is an idiom that originated in the United States. It refers to someone who doesn't participate in an action but still makes judgments about it. The phrase takes its meaning from the fact that a quarterback is the player in the American sport of football who controls much of the game. An "armchair quarterback" is someone who offers his or her opinion on something without actually being a part of it. It is an idiom that has branched out from the sports world to encompass many other aspects of life.
One of the most popular sources for idioms, phrases that gain meanings somewhat different from their literal definitions through popular use, is the sports world. Each sport is associated with specific terminology, and certain popular sports terms and phrases are used in contexts other than sports to illustrate ideas or points. Football, which is the dominant sport in the US in terms of fan interest, is a particularly rich source of idioms such as "armchair quarterback."
To understand the meaning of "armchair quarterback," it is crucial to understand the role a quarterback plays on the football field. The quarterback touches the ball on practically every offensive play, decides to whom the ball will be given by either passing it or handing it off, and often is in charge of calling the plays that the offense will run. It is generally regarded by football experts to be the most important position on a team.
Of course, the irony of this phrase is that someone sitting in an armchair is a passive viewer and not an active participant in the event in question. As a result, the phrase is often used by the speaker to describe someone in a snide and sarcastic manner. For an example of how the idiom is used, someone might say, "He thinks he knows all the answers, but he is nothing but an armchair quarterback."
As an idiom, "armchair quarterback" has grown from being used in just a football context to the point where it is used in many different applications. For example, political pundits who appear on television are often derisively described in this manner if they've never taken an active role in politics themselves. In general, anyone who has strong opinions on a topic but either doesn't act on those opinions or only offers them after the action is completed can be labeled in this way.