In the game of hopscotch, a player employs an object to throw into each successive chalked box he/she is trying to pass through. (Some schoolyards had painted hopscotch lines, a real challenge for smaller, younger players.) That object, his/her token, is a player's potsy. In some games potsies required the approval of all players as some potsies were perceived to give a player an unfair advantages.
Player 1: I'll use my wallet as a potsy.
Player 2: Too flat and heavy. It's too much of a sure thing. Pick something else.