When it is time to pay the piper it is time to accept the consequences of a thoughtless or rash action. Or the phrase can mean that it is time to fulfill a responsibility or promise, usually after the fulfillment has been delayed already. Almost always the phrase is used with a pejorative connotation.
The phrase comes from the fable of the Pied Piper of Hamelin. According to the myth, a piper was hired to clear out the rats from the village of Hamelin. After he did so (by playing a song on his pipes), he was not paid for his work. His revenge for the lack of payment was to steal all the children of the town. The moral of the story was to pay the piper, or keep up your half of the bargain.
As a phrase, the verb is conjugated through all its forms.
A related phrase is he who pays the piper calls the tune. Somewhat self-explanatory. The phrase means that the one who is footing the bill gets to make the decisions. This is sometimes used to mean that the wealthy have all the power.
The result is we overeat at Thanksgiving, overindulge in December without regard for calories, and then go skidding into the new year generally carrying a few extra pounds. January is when we pay the piper. Nobody brings treats into the office this month and we are surrounded by a sea of salads in the lunchroom. [Huffington Post]
Two, the way the parties are funded, it’s only a few people that fund the parties, and he who pays the piper dictates the tune. [All Africa]
We are already paying the piper for our collective denial of responsibility; polluted water and air, drying aquifers, dead zones in our oceans, global warming, loss of species and habitat, human suffering, ….. on and on. [Ravalli Republic]