打退堂鼓 literally means 'to beat the retreat drum' — an ancient practice where a drum was beaten to signal retreat from court or battle. It is a verb-object idiom (离合词) used metaphorically to mean 'to back out or give up halfway through something'. Since it's a separable verb-object, you can insert aspect markers (了, 过, 着) or time words between 打 and 退堂鼓, e.g. 打了退堂鼓 (backed out) or 打起了退堂鼓 (started thinking about backing out). Use it when describing someone withdrawing from a plan, project, or commitment.