出典:Wiktionary
May have originated with the game of poker, in which a marker or counter, frequently in frontier days a knife with a buckhorn handle, was used to indicate the person whose turn it was to deal. If the player did not wish to deal he could pass the responsibility by passing the "buckhorn" or "buck", as the marker came to be called, to the next player.
pass the buck (三人称単数 現在形 passes the buck, 現在分詞 passing the buck, 過去形および過去分詞形 passed the buck)