Learner's definition of PIPER
: a person who plays a pipe or the bagpipes
pay the piperinformal
1 : to pay the cost of something
We have to do what they say because they're the ones who are paying the piper.
You know what they say: he who pays the piper calls the tune. [=the person who pays for something controls how it is done]
2 chiefly US : to pay money or suffer in some way because of something you have done