Definition of In Their Flip-flops

What is the meaning of the term 'In their Flip-flops' when it comes to the game of football?

When someone says that a team is already in their flip-flops it means that the players have nothing left to fight for in the season and are already on summer vacation in their minds. A good example of a team that is already 'in their flip-flops' this season (2014/15) is English Premier League's Stoke City. With ten or so games left, Stoke are positioned in mid-table, safe from relegation and with UEFA Champions League and Europa League out of their reach. Stoke are also out of the domestic cup competitions.

When a team is already 'in their flip-flops' they might not fight as hard and may stray away from dangerous challenges, situations etc.