To pass the buck is to avoid responsibility by transferring it to someone else. A manager who passes the buck when a project fails blames their team rather than taking accountability. A government that passes the buck on a social problem keeps redirecting the issue to another department. The phrase implies a failure of leadership or courage.
The phrase comes directly from 19th century American poker. The 'buck' was a marker — typically a knife with a buckhorn handle, or later a silver dollar — placed in front of the player whose turn it was to deal. If a player did not wish to deal, they could 'pass the buck' to the next player, literally sliding the marker forward. Dealing carried responsibility for managing that hand of cards.
President Harry Truman made the phrase internationally famous by reversing it. He kept a sign on his Oval Office desk that read: 'The buck stops here.' The sign was a direct reference to the poker phrase and communicated that Truman, as President, would not pass responsibility to others. He would be the final decision-maker and would own the consequences. The sign is now on permanent display at the Truman Presidential Library.
Early bucks were typically a knife with a handle made from a buck's horn — hence the name. By the later 19th century, silver dollars were also used as the dealer marker. The buckhorn knife was common in frontier card games.
The dealer in many poker variants controls the flow of the game — they set the pace, resolve disputes about rules, and are responsible for ensuring the game runs fairly. Passing the buck removed you from this position of responsibility and authority.